.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

[markwallace-microsoft]

Unrelated integration test failure

[xUnit.net 00:02:09.98]     SemanticKernel.IntegrationTests.Connectors.AzureOpenAI.AzureOpenAIChatClientRequiredFunctionChoiceBehaviorTests.SpecifiedInPromptInstructsConnectorToInvokeKernelFunctionAutomaticallyAsync [FAIL]
[xUnit.net 00:02:09.98]       Polly.Timeout.TimeoutRejectedException : The operation didn't complete within the allowed timeout of '00:00:30'.
[xUnit.net 00:02:09.98]       ---- System.Threading.Tasks.TaskCanceledException : The operation was canceled.
[xUnit.net 00:02:09.98]       -------- System.IO.IOException : Unable to read data from the transport connection: Operation canceled.
[xUnit.net 00:02:09.98]       ------------ System.Net.Sockets.SocketException : Operation canceled