Prepared statement support

[zachmu]

This PR implements postgres prepared statements with support for $1 style parameters in query strings. If this approach is reasonable, the next step would be to add support for additional types and lots more tests.

Also missing is support for the client manually specifying the types of parameters in a Parse message, which is required for certain statements where the type information cannot be determined based on referenced columns.

Companion PRs:

dolthub/go-mysql-server#2239
dolthub/vitess#299

[Hydrocharged]

Great work! Really glad you're tackling this. Small nitpick comments, but the core logic looks good to me! Not straightforward, however I think that's just the nature of the beast.