.Net: Optimize and clean up the PgVector provider

[roji]

• 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