mysql: Pass mysql.Conn through {Hash,PlainText,Caching}Storage interfaces

[mattrobenolt]

The TLS client certificates can simply be derived based on the
mysql.Conn passed through, but passing through the actual Conn, gives us
access to Conn.ClientData or anything else an Auth{Method,Server} might need to
implement.

It's arguable that if we need access to the lower level Conn, we could
simply implement the actual HandleAuthPluginData instead of these
*Storage interfaces, and that's true. But not all AuthMethods can get
away with this because they need to write to the Conn's connection using
private functions.

For example, the mysqlCachingSha2AuthMethod needs to write more data
on the socket after calling UserEntryWithCacheHash, so to write a new
UserEntryWithCacheHash that needs data from mysql.Conn, I'm left
wtih no other options since I can't vendor or reimplement the rest of
the logic without having access to the private functions/methods within
the mysql package.

Checklist

• Should this PR be backported? (no)
• Tests were added or are not required (no, existing tests were modified)
• Documentation was added or is not required (In changelog probably?)

[deepthi]

@mattrobenolt welcome to vitess and thank you for the contribution!