Auth plugin caching sha2 password

[wenjoy]

It's related about #16

Currently it works for both mysql 8.0 and mysql 5.6 based on my test.
But it just works. There is still work left to completely support mysql8.0. In further, we need to:

• support negotiation of auth method if mismatched
• used configured public key
  and others I doesn't figure out now.

Anyway, please help to check this PR and give me feedbacks. Thanks!

[manyuanrong]

Great job!
At present, this feature needs to be verified by testing

[manyuanrong]

For the feature of configuring publicKey, it may be good to do it in the new PR, because testing it requires more changes to CI and test cases

[wenjoy]

For the feature of configuring publicKey, it may be good to do it in the new PR, because testing it requires more changes to CI and test cases

Agree, should we also add these plans to our TODO.

[manyuanrong]

Agree, should we also add these plans to our TODO.

Of course this is the best

[invisal]

It works on mysql8 with caching sha2 enabled on my computer.

[robinbertram]

This PR also solved the "Got packets out of order" problem I had. Great work!

[suchislife801]

"It works on mysql8 with caching sha2 enabled on my computer."

How? Where? How's this working? Where do I find it working?