improve out of the box psycopg2 support

[tychoish]

Presently, when attempting to connect to glaredb over the postgres protocol with the psycopg2 python module, unless connection.autocommit = True, psycopg2 will attempt to wrap all cursor operations in a transaction, sending BEGIN TRANSACTION queries to glaredb. As a result, queries throw an exception and users see the PGRES_EMPTY_QUERY error. Setting autocommit to true, disables the "wrap in a transaction" behavior.

The solution to this is somewhat difficult. We could (and should) improve the error message so users know that they were sending a transaction when they didn't mean to. It is (probably) correct to error early when we see a transaction that we shouldn't.

Uncovered by the testing in #2258.

[universalmind303]

related to #2402

[tychoish]

@scsmithr and I discussed this today and we decided:

• we should document that the postgres protocol only does not support transactions.
• glaredb should send a notice on the pg protocol when someone uses a trasaction statement, but otherwise continue.
• cloud should print notices via the postgres protocol
• in local mode, and for RPC/flight sql, transaction statements should be errors.

Some further carve outs.

• we may also want to say that we may eventually provide transactions over the postgres protocol in the future.
• it may make sense to still reject write operations inside of transactions.
• it might make sense to reject some higher transaction levels too.