…orkflows

Reworked how code is checked out in GitHub Workflows.
Inside '${{github.workspace}}' we now have:
- 'src' (for the main project source tree)
- 'aws-sdk-cpp' (for AWS SDK C++ source tree with submodules)

All Github Workflows 'checkout' actions are now instructed to fetch tags as well
('fetch-tags: true'). For our own code we also fetch full commit history
('fetch-depth: 0').

Reworked the way how we form labels for matrix jobs:
'<BUILD_TYPE>-<COMPILER_LABEL>', where
'<BUILD_TYPE>' is either 'Debug', 'RelWithDebInfo', or 'ASan' and
'<COMPILER_LABEL>' is either 'clang17' or 'gcc13'.
These labels are used in directory names and in cache keys.

Changed the way how Boost libraries are cached: instead of caching the binary
tarball ('.tar.bz2'), the unpacked directory is cached. This helps to eliminate
compressing already compressed data. Caching key now also includes the library
version (currently, 'boost-libraries-1-83-0').

Added a new step that shows '${{github.workspace}}' and '${{runner.temp}}' for
diagnostic purposes.

Added a new step that shows the content of the following directories:
- '${{github.workspace}}'
- '${{runner.temp}}'
- '${{runner.temp}}/deps'
for diagnostic purposes.

GitHub Workflows now builds AWS SDK C++ libraries.
* Currently the selected version is 1.11.286 but it is configurable via
  ${{env.AWS_SDK_CPP_MAJOR}}, ${{env.AWS_SDK_CPP_MINOR}},
  and ${{env.AWS_SDK_CPP_PATCH}} variables.
* The code is checked out from the 'aws/aws-sdk-cpp' GitHub repo (recursively
  with submodules) by '1.11.286' tag.
* CMake is instructed to build only static versions of the libraries
  with all the dependencies ('-DBUILD_SHARED_LIBS=OFF' and
  '-DFORCE_SHARED_CRT=OFF').
* The same compiler and build configuration as for the main project are used to
  build the libraries ('-DCMAKE_BUILD_TYPE=...', '-DCMAKE_C_COMPILER=...', and
  '-DCMAKE_CXX_COMPILER=...').
* C++ standard is set to 'c++20' as in the main project ('-DCPP_STANDARD=20').
* In order to instruct AWS SDK C++ to use 'libc++' standard library
  implementation (for 'clang' compiler), '-DCMAKE_CXX_FLAGS_INIT=-stdlib=libc++'
  option is passed manually for such configurations.
* For Address Sanitizer builds '-DENABLE_ADDRESS_SANITIZER=ON' option is added.
* Unity build is enabled ('-DENABLE_UNITY_BUILD=ON').
* Building/running tests is disabled ('-DENABLE_TESTING=OFF' and
  '-DAUTORUN_UNIT_TESTS=OFF').
* Currently only 's3-crt' component is build ('-DBUILD_ONLY=s3-crt').
* The full specification of these options is available at
  https://docs.aws.amazon.com/sdk-for-cpp/v1/developer-guide/cmake-params.html
* 'conan' package manager recipe
  https://github.com/conan-io/conan-center-index/blob/master/recipes/aws-sdk-cpp/all/conanfile.py
  and 'vcpkg' port
  https://github.com/microsoft/vcpkg/blob/master/ports/aws-sdk-cpp/portfile.cmake
  were taken as examples.

The libraries are built in the
'${{github.workspace}}/aws-sdk-cpp-build-${{matrix.config.label}}' directory.
The libraries are installed into the
'${{runner.temp}}/deps/aws-sdk-cpp-install-${{matrix.config.label}}' directory.
The build results (the content of the installation directory) are cached under
the 'ws-cpp-sdk-libraries-<SDK_VERSION>-<BUILD_TYPE>-<COMPILER_LABEL>' key
(e,g, 'aws-cpp-sdk-libraries-1-11-286-RelWithDebInfo-clang17'). This allows to
skip building AWS SDK C++ libraries every time a new PR is created. The rebuild
is expected to happen only when the SDK version is updated or a new version of
the compiler is added.

'-DCPP_STANDARD=20' CMake configuration option is now also added for the main
project to synchronize with AWS SDK C++ libraries.

Main 'CMakeLists.txt' file now tries to find AWS SDK C++ libraries via
'find_package()'. Developers are now expected to pass
'-DCMAKE_PREFIX_PATH=<AWS_SDK_CPP_INSTALL_DIR>' CMake option when they configure
the project, where '<AWS_SDK_CPP_INSTALL_DIR>' should point to the AWS SDK C++
installation directory.

Main application target now depends on 'aws-cpp-sdk-s3-crt'.

Added a stub for the second implementation of the
'binsrv::basic_storage_backend' interface, called 'binsrv::s3_storage_backend',
which in future will provide basic operations for working with AWS S3 storage.

'binsrv::storage_backend_factory' factory class extended with a new concrete
implemetation. Now:
- for 'fs' we return an instance of 'binsrv::filesystem_storage_backend'
- for 's3' we return an instance of 'binsrv::s3_storage_backend'.

'binsrv::basic_storage_backend' interface extended with one more abstract method
'get_description()':
- 'binsrv::filesystem_storage_backend' ('fs') implementation returns
  "local filesystem"
- 'binsrv::s3_storage_backend' ('s3') implementation returns
  "AWS S3 (SDK <SDK_VERSION>)" ('<SDK_VERSION>' is populated from the
  'Aws::SDKOptions' struct initialized via 'Aws::InitAPI()' call).

Main application extended with writing 'get_description()' output into the log.