.Net: Optimize and clean up the PgVector provider (#12441)

* Removed the intermediate `Dictionary<string, object?>` in
serialization/deserialization. Like with SQL Server and SQLite, we now
deserialize directly from NpgsqlDataReader, etc.
* Removed PostgresSqlCommandInfo, which was basically the same thing as
NpgsqlCommand. Code now simply instantiates NpgsqlCommand directly,
instead of instantiating PostgresSqlCommandInfo and then creating an
NpgsqlCommand from that.
* Removed PostgresDbClient - command creation is now done in
PostgresCollection directly (actual SQL is still generated
PostgresSqlBuilder for unit testing, at least for now).
* Moved the reference-counting functionality of PostgresDbClient to a
minimal `NpgsqlDataSourceArc` ("atomic reference counter"), which does
only this.
* Various other minor cleanups.

Closes #11123

Author: roji