Support Google Cloud Spanner (GoogleSQL dialect)

[rafi-rr]

This PR adds support for Spanner as an additional database option in parallel to BigQuery and PostgreSQL. It accesses a Spanner instance and then is capable of using all databases within that instance as data sources. Furthermore, the embeddings database is defined separately as a separate database.

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[rafi-rr]

Thanks @5Y5TEM Taking care of this now.

[rafi-rr]

@5Y5TEM Verified on my end following conflict resolution.

[5Y5TEM]

@rafi-rr I have resolved all the issues and merged your code in the spanner branch: https://github.com/GoogleCloudPlatform/Open_Data_QnA/tree/spanner

Once the team is done with testing, we will merge it to main.

Thank you for contributing! I will resolve this PR for now.