feat: implement query options versioning support

[larkee]

Adds support for QueryOptions.

The QueryOptions are set in one of three ways (in ascending order of precedence):

• As an argument to the Client constructor.
• Setting the SPANNER_OPTIMIZER_VERSION env variable.
• On a per-query basis.

If the backend implements query hints, for example, to instruct the backend which query optimizer version to use, these query hints will also automatically be supported.

[skuruppu]

Not sure why there's a diff for _get_spanner_emulator_host in this PR.

[larkee]

Both emulator support and query options support rely on calling os.getenv which means it's more difficult to set up the mock. The os.getenv call for emulator is made in _get_spanner_emulator_host which means mocking that function instead is much more clear and avoids setting up theos.getenv mock.

[hengfengli]

probably use OPTIMIZER_VERSION_ENV_VAR

[hengfengli]

Major concerns:

1. Does query_options support both ExecuteSqlRequest.QueryOptions and dict everywhere? It needs to be clear in the doc comment.
2. Does partition_query support query_options as well?
3. Nits: the merge function can be more generic for future extension. This can be done in the future when we add new fields in query_options.

For the rest, LGTM, 👍

[skuruppu]

Just to confirm that we're going with SPANNER_OPTIMIZER_VERSION to be consistent with the query hint name. I think your implementation already uses this name but just wanted to make sure.

[larkee]

@hengfengli partition_query does not support query_options as it does not call executeSql or executeStreamingSql

[hengfengli]

@hengfengli partition_query does not support query_options as it does not call executeSql or executeStreamingSql

Actually, I mean this method generate_query_batches. You have done it already.

[hengfengli]

LGTM 👍