support for custom ExecutorProvider in SpannerOptions.Builder

[pravsingh]

Is your feature request related to a problem? Please describe.
I would like to configure ExecutorProvider in SpannerOptions so can control the executor configs. Currently the Builder doesn't provide any setter for it. Due to this, the asyncExecutorProvider is null and hence the below code always falls back to the default provider:

SpannerImpl(SpannerRpc gapicRpc, SpannerOptions options) {
...

    this.asyncExecutorProvider =
        MoreObjects.firstNonNull(
            options.getAsyncExecutorProvider(),
            SpannerOptions.createDefaultAsyncExecutorProvider());
...
}

Describe the solution you'd like
Add setter in SpannerOptions.Builder class.

Describe alternatives you've considered
So far found no alternative.

Additional context
Discussed about this issue at discussion

[thiagotnunes]

@olavloite Could you take a look at this? I think you were already following this issue before.