Implement support for cleartext authentication plugin

[wildmaples]

Resolves: https://github.com/trilogy-libraries/trilogy/issues/157 Redo of: https://github.com/trilogy-libraries/trilogy/pull/168

The server doesn't request for cleartext plugin in the first handshake as it is not a default plugin. The client should only provide cleartext password when the server requests an auth switch to it, and the client has enable_cleartext_plugin option enabled. If the client does not have cleartext auth plugin enabled, we will raise a Trilogy::AuthPluginError.

This is the authentication flow for a server that uses the cleartext client side plugin (https://github.com/trilogy-libraries/trilogy/pull/168#discussion_r1548811735):

Server sends handshake, with auth plugin caching_sha2_password Client sends handshake response, still with auth plugin caching_sha2_password Server sends auth switch request, now with auth plugin mysql_clear_password Client sends auth switch response with just the cleartext password and nothing else Server sends OK and we are connected

This PR introduces these changes:

• Supports cleartext auth plugin!
• Raises when the server auth switches to a plugin that Trilogy does not support. Note that at the moment Trilogy supports mysql_native_password, caching_sha2_password and now mysql_clear_password. This is the recommended behaviour according to mysql docs "Authentication Method Mismatch".
• Adds a test dependency on this mysql test server plugin. It's unclear if the file will stick around in the long term but seeing that it is only a test dependency and makes our lives easier than the alternative testing options, we are inclined to just use it for the time being.

cc: @ngan @composerinteralia

[ngan]

I created an issue on the MySQL docker repo to see if they'd be open to building and tagging a mysql-community-test variant that we can use. If they do, we can switch to that and get the test auth plugin for free. If they don't, we can keep what we have (checked-in plugins for various mysql version), or build our own MySQL image.

https://github.com/docker-library/mysql/issues/1040

[composerinteralia]

Would you mind squashing your commits?

I do like a clean git history, but we haven't historically been as opinionated about this as (for example) Rails.

[composerinteralia]

Thank you!