feat: Support Leader Aware Routing

[SandeepTuniki]

This PR implements Leader Aware Routing (LAR) for the Ruby Spanner client. For background design, see http://shortn/_5ecDWu2XD9 (internal)

Note:

• LAR is enabled by default. But we provide an option to users to disable it during client initialization.
• Admin RPCs in the handwritten Ruby client are deprecated, so LAR isn't supported for these RPCs.

Tests

• Majority of the testing is done through unit tests.
• For acceptance tests, a basic smoke test is added to validate querying without LAR. All the existing acceptance tests run with LAR enabled by default.

[NivedhaSenthil]

I am not sure why do we need a separate place to handle this as the context for calling these methods are still with the respective methods why not just default the values in place itself ?

[SandeepTuniki]

Mostly to facilitate future changes easily. I first used the default values in place. But soon I found that these "magical constants" were spread across the codebase, making it harder to spot any specific values in the future. So I moved them to this helper module.

[dazuma]

I think that's fine. However, the downside of doing this is that these values are separated from their context so it's less clear what they are for. So I recommend making sure we document each of the methods here with what the value is used for (and maybe under what circumstances it might change in the future).

[SandeepTuniki]

I added description at the top of the module explaining the context, and when the values can differ.

I avoided commenting for individual methods to avoid duplication. But let me know if the present documentation doesn't suffice. I'll make changes.

[dazuma]

A few minor comments, otherwise looks good.

[dazuma]

I think that's fine. However, the downside of doing this is that these values are separated from their context so it's less clear what they are for. So I recommend making sure we document each of the methods here with what the value is used for (and maybe under what circumstances it might change in the future).