sha256_password auth support

[julienschmidt]

Description

Fixes #625

It also adds the ability for caching_sha2_password authentication to use a local copy of the servers pub key.

This PR depends on #807, which has to be merged first (This PR will then be rebased and the changeset will be a lot smaller)

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[methane]

I don't like registry.
Now we have Connector interface. How about adding ServerPubKey only to Config object?

[julienschmidt]

It's consistent with the existing code and we still support Go versions that do not support the Connector interface.

Even with the Connector interface it is still useful to be able to serialize a config to a DSN string. That is only possible if the keys etc are addressable by an identifier.

[methane]

OK. But I don't want to use DSN and registry anymore.
I want Config.ServerPubKey *rsa.PublicKey.

Config.ServerPubKeyName string can be added separately for Config.FormatDSN().

[julienschmidt]

That would require more changes to properly handle configurations that cannot be formatted as a DSN.
If you insist, I'm open to changing the exported struct attribute name to Config.ServerPubKeyName but adding an exported *rsa.PublicKey cannot happen has part of this pull request.

[julienschmidt]

Using a config to format a DSN is by the way currently its only usage. I don't plan to add support for the Connector interface before the 1.4.0 release. This will require a few more thoughts and changes as well as proper testing for which we don't have time if the 1.4.0 release should happen soon.

Currently a config would thus be unusable as soon as someone adds a *rsa.PublicKey directly.

[methane]

Then, how about ServerPublicKeyPath?
It's consistent with mysql --server-public-key-path commandline option.

[julienschmidt]

It isn't a path but an identifier (name) for a registered key.
Unlike the mysql command line tool we never read any (file-)path.

[methane]

OK. While I think registries are ugly workaround for DSN limitation, we can't deprecate it yet...