Implemented AWS S3 storage backend

[percona-ysorokin]

Current implementation accepts storage URIs in the following form: 's3://[<access_key_id>:<secret_access_key>@]<bucket_name>[.]/'. In case '' is not specified explicitly, it will be auto-detected from the location of the '<bucket_name>' bucket.

As AWS S3 does not have a direct way to append data to an existin object, we use the following strategy for resuming stream operations.

• Upon opening a stream (via 'do_open_stream()' method) for an object that already exists on S3 (in case we run the utility on a storage that has already been initialized), we download the content of this object into a temporary file.
• All writes (data appends) requested by the 'do_write_data_to_stream()' method will be performed to this temporary file.
• When 'do_close_stream() method is called, we upload the content of this temporary file back to S3 (overwriting the content of the existing object).

Re-designed 'binsrv::s3_storage_backend' class. Introduced internal 'aws_context' class that follows the 'pimpl' idiom, so that the main class include file 'binsrv/s3_storage_backend.hpp' does not depemnd on any '<aws/*>' headers.

'binsrv::filesystem_backend_storage' class now explicitly specifies required combinations of the 'std::ios_base::[in | out | binary | trunc | app]' flags in full for all internal 'std::ifstream' / 'std::ofstream' / 'std::fstream' objects.

'binsrv::storage' class destructor now tries to call 'backend_->close_stream();' in order to flush stream state to the storage backend in case of normal / exceptional shutdown.

'binsrv::basic_storage_backend' class extended with additional method 'is_stream_open()' indication whether the object is in a state between 'open_stream()' and 'close_stream()' calls.

'open_stream()' method in the 'binsrv::basic_storage_backend' class now accepts one additional parameter indicating an intent to either create or append a storage stream explicitly.

Similarly to 'easymysql::core_error' added 'binsrv::s3_error' exception class with its own 's3_category()' error category ('std::error_category').

Similarly to 'easymysql::raise_core_error_from_connection()' added 'raise_s3_error_from_outcome()' helper function which throws an exception with error info extracted from 'Aws::S3Crt::S3CrtError' (an error-path alternative of almost any 'S3Crt' call outcome).

'easymysql::raise_core_error_from_connection()' helper function extended with additional 'user_message' parameter.

Main application now prints 'successfully shut down' to the log at the end of execution.

'binsrv' MTR test case extended with additional logic that allows to use both 'file://' and 's3://' as backend storage providers (the choice depends on whether 'MTR_BINSRV_AWS_ACCESS_KEY_ID' / 'MTR_BINSRV_AWS_SECRET_ACCESS_KEY' / 'MTR_BINSRV_AWS_S3_BUCKET' environment variables are set or not).

Added extra precautions for accidentally leaking AWS credentials - we now temporarily disable MySQL general query log to make sure that 'AWS_ACCESS_KEY_ID' / 'MTR_BINSRV_AWS_SECRET_ACCESS_KEY' will not appear in the recorded SQL queries.

Added 'diff_with_storage_object.inc' MTR include file that can compare a local file with an object from backend storage (either 'file' or 's3').

Added more instructions on how to make 'binsrv' MTR test case use AWS S3 as a storage backend in 'mtr/README'.

Added '.clang-format' file to 'mtr' directory to exclude MTR test cases and include files from being processed by 'clang-format'.