Add x-use-transaction mode to postgres driver for pg-proxy compatibility

[areazus]

Session-level advisory locks (pg_advisory_lock) are not supported by
pg-proxies such as Orgstore's pg-proxy, which route each statement to a
potentially different backend connection. This makes it impossible to
guarantee that Lock and Unlock execute on the same connection.

Add a new x-use-transaction URL option (UseTransaction config field) that
switches the driver to transaction-scoped advisory locks
(pg_advisory_xact_lock). In this mode, a single transaction spans the
advisory lock acquisition, migration SQL, and version table update, with
the lock released automatically on commit or rollback. This is compatible
with pg-proxy deployments since all statements are issued within one
transaction on a single connection.

Key changes:

• Add withExistingOrNewTxRollbackOnErr to manage a shared *sql.Tx,
  including proper rollback and cleanup of p.tx on error
• Add executeExecContext, executeQueryContext, executeQueryRowContext
  wrappers that route through the shared transaction when enabled
• Update Lock/Unlock, SetVersion, Run, Drop, and ensureVersionTable to
  use the new wrappers
• Fix transaction leak when SET LOCAL statement_timeout fails on startup
• Ensure p.tx is cleared after rollback so subsequent calls start fresh
• Return nil from Unlock when the transaction is already done (rolled
  back by a failed migration), since the lock is already released
• Update dktesting cleanup client to use API version negotiation
• Document the feature and its limitations (CONCURRENTLY, VACUUM, etc.)

Co-Authored-By: Claude Sonnet 4.6 (1M context) noreply@anthropic.com

[coveralls]

coverage: 54.645% (+0.2%) from 54.432%
when pulling 257c1fd on DataDog:feat/postgres-use-transaction
into 257fa84 on golang-migrate:master.