feat: Support for Spanner ReadLockMode

[purva9413]

This PR adds in the ability to set ReadLockMode as a nested option under ReadWrite transaction options.

TESTED=All acceptance and unit tests pass

[aandreassa]

Added a comment on documentation. Could we add an integration test just to make sure the server is accepting the new argument?

[purva9413]

Added a comment on documentation. Could we add an integration test just to make sure the server is accepting the new argument?

I have added in a do not merge tag which I will remove when we have the feature set up on the backend. Not sure if it makes sense adding integration tests for every new option we introduce in the client library for the purpose of testing whether the backend accepts it though. We can have one integration test (maybe in a seperate PR) to check all TransactionOptions are set and sent correctly to the backend. Thoughts?

[aandreassa]

Some nits!

Also, I am OK with adding a single integration test that sets fields in transaction options. Would it be difficult to do it in this PR?

It should be helpful to double check that we are sending the correct format for the request.

[aandreassa]

Looks great, thanks!