feat: pluggable auth plugins

[barakalon]

What do these changes do?

First off - thanks for all the work on this package :)

This PR introduces a framework for auth plugins. This will make it easier to add auth plugins.

Why?

I need to implement the kerberos plugin. We also use this client with services that only imitate the mysql server (i.e. starrocks and proprietary services based on mysql-mimic), and I might need proprietary auth plugins.

Are there changes in behavior for the user?

Nope. All the existing auth_plugins should work as-is.

Related issue number

n/a

Checklist

• I think the code is well written
• Unit tests for the changes exist
• [] Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

This has tests in the sense that existing tests don't fail. But I don't think the existing tests for auth are that extensive, so it might warrant some more testing.

As for documentation - I'm not sure this should be documented just yet.

[barakalon]

And kerberos authentication: https://github.com/barakalon/aiomysql/blob/barak/kerberos/aiomysql/auth.py

[barakalon]

@Nothing4You any thoughts on this one?

[Nothing4You]

Hi @barakalon,

thank you for the PR, unfortunately it'll probably be a few more weeks until I'll have time to review this.

[barakalon]

Sounds good 👍