feat: add support for QueryOptions

[olavloite]

Adds the ability to set QueryOptions when running Cloud Spanner queries. For now, only setting the query_optimizer_version is added.

QueryOptions can be configured through the following mechanisms:

1. Through the SPANNER_OPTIMIZER_VERSION environment variable.
2. At Database level using spanner.instance('instance-name').database('database-name', sessionPoolOptions, queryOptions).
3. At query level using ExecuteSqlRequest.queryOptions.

If the options are configured through multiple mechanisms then:

1. Options set at an environment variable level will override options configured at the Database level.
2. Options set at a query-level will override options set at either the Database or environment variable level.

If no options are set, the optimizer version will default to:

1. The optimizer version the database is pinned to in the backend.
2. If the database is not pinned to a specific version, then the Cloud Spanner backend will use the "latest" version.

[skuruppu]

LGTM

If you can update the PR description to:

1. List the precedence
2. Mention the env var name
   that would be great.

[skuruppu]

I assume these are auto-generated changes?

[olavloite]

Yes, although it could be that this change is only 'generated' because of the way I manually generated the proto changes. It should however not make any difference for this PR once the actual proto change has been merged.

[skuruppu]

It seems that the generated code change is on master so I think you should be able to revert this file.

[olavloite]

LGTM

If you can update the PR description to:

1. List the precedence
2. Mention the env var name
   that would be great.

Done.

[skuruppu]

Thanks for the changes. One more request @olavloite, would you be able to upload any samples you have? I'm not sure whether it should be in this PR or in a separate one but I'll let you decide.

[olavloite]

Thanks for the changes. One more request @olavloite, would you be able to upload any samples you have? I'm not sure whether it should be in this PR or in a separate one but I'll let you decide.

I've added the samples to this PR.

[skuruppu]

Thanks @olavloite. Please merge in the change after you've resolved the merge conflicts.