Don't store plaintext passwords

[gdsmith]

Rather store hashed passwords in the way mysql server does.

See #1037

here we:

• update CredentialProvider to return new Credential struct
• Credential includes the plugin that the user was created with
• update InMemoryProvider to handle hashing of passwords and add default auth method to make usage backwards compatible
• update server authentication to use mysql server methods of comparing hashes rather than relying on having the plaintext password available
• rework the password negotiation to switch plugin type to match the stored credentials
• add hashing and comparison functions for the above where missing from existing libraries

[gdsmith]

Code is fully working as much as I can see but I am seeing failures with the TestCachingSha2Cache tests. I don't fully understand if it's a client problem or a change I've introduced means the server is pulling those credentials multiple times when it didn't previously. I did try debugging and it seemed like the cache was empty even if I confirmed the cache was actually written. Would love some feedback, and some help with the failing tests if at all possible.

[gdsmith]

The changes to the code is what has broken the test, we are now getting the password to confirm the auth method. I think the code changes break the way you are checking if we are using the cache response, but I'm not sure how to update the test to ensure we are using the cache (though through debugging I can confirm that it does)