Fail authentication when server requests unknown authentication plugin

[clue]

This changeset ensures we properly fail authentication when the server requests an unknown authentication plugin:

$ php examples/01-query.php 
Error: Connection to mysql://test:***@localhost/test failed during authentication: Unknown authentication plugin "caching_sha2_password" requested by server

The feature itself is mostly limited to better debugging output to give downstream consumers a better understanding what's going on. For reference, the old error looks like this:

$ php examples/01-query.php 
Error: Connection to mysql://test:***@localhost/test failed during authentication: Client does not support authentication protocol requested by server; consider upgrading MySQL client (EACCES)

On top of this, this is really done in preparation to add proper authentication plugins, in particular the caching_sha2_password authentication mechanism used by default on MySQL 8+ as discussed in #112. Once this PR is merged, I'll file a follow-up to add caching_sha2_password support.

The affected code has 100% code coverage and has been tested against a number of (older) MySQL server versions. The tests confirm it should continue to work for legacy servers not using authentication plugins or newer MySQL server versions using the existing mysql_native_password authentication. We should probably follow-up on PR #196 to add more MySQL server versions to our test matrix.

Marking this as WIP until the unrelated test failures are addressed via #205. Merged.

Builds on top of #205
Refs #112

[clue]

Rebased now that #205 has been merged, this is now ready for review

[WyriHaximus]