refactor(pluginservers): code refactor & testing

[gszr]

Context

The overall goal of this commit is to refactor the external plugins
implementation, with the following goals in mind:

• Make plugin server code more approachable to unfamiliar engineers and
  easier to evolve with confidence
• Harden configuration; ensure configuration defects are caught before
  Kong is started
• Extend testing coverage

This is related to ongoing work on the Go PDK, with similar goals in
mind.

Summary

This commit implements the following overall changes to plugin server
code:

• Move configuration related code into conf loader, so that configuration
  loading and validation happens at startup time, rather than lazily, when
  plugin data is loaded or pluginservers are started. Add tests for
  current behavior.

• Move process-management code - for starting up plugin servers as well
  as querying external plugins info - into the process.lua module.

• Introduce a kong.runloop.plugin_servers.rpc module that encapsulates
  RPC initialization and protocol-specific implementations. This further
  simplifies the main plugin server main module.

• Factor exposed API and phase handlers bridging code into a new plugin
  module, which encapsulates an external plugin representation, including
  the expected fields for any Kong plugin, plus external plugin-specific
  bits, such as the RPC instance. Part of this external plugin-specific part
  is the instance life cycle management. With this structure, the kong.runloop.plugin_servers
  main module contains only general external plugin code, including a list
  of loaded external plugins, and associated start/stop functions for
  plugin servers.

Testing

This commit also implements the following improvements to tests:

• Restructure fixtures to accommodate new external plugin servers --
  namely, targeting for now in the existing Python and Javascript
• Add new test cases for external plugins:
  • External plugin configuration: add test cases for current behavior;
    in particular:
    • Fail if no query_cmd is provided;
    • Warn if no start_cmd is provided - this is by design, as
      external plugins servers can be managed outside of Kong
  • Plugin server start / stop - for both Go and Python plugins
  • External plugin info querying for both Go and Python plugins
  • External plugin execution - for both Go and Python plugins<!--

KAG-2496

[team-gateway-bot]

Cherry-pick failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git remote add upstream https://github.com/kong/kong-ee
git fetch upstream master
git worktree add -d .worktree/cherry-pick-12858-to-master-to-upstream upstream/master
cd .worktree/cherry-pick-12858-to-master-to-upstream
git checkout -b cherry-pick-12858-to-master-to-upstream
ancref=$(git merge-base 9ae36382db4b9485f8889b8cd80d66a0885c86e7 be4049980e16448b609a00b6a1f7453013d57fcf)
git cherry-pick -x $ancref..be4049980e16448b609a00b6a1f7453013d57fcf