impl(bigtable): MutateRowsLimiter can do async

[dbolduc]

Part of the work for #12959

I considered having the ThrottlingMutateRowLimiter hold a CompletionQueue and Options const&, but I think it is nicer to just pass the async sleeper (std::function<future<void>(duration)>) in.

---

I considered only having an async sleeper, and implementing the following:

void ThrottlingMutateRowsLimiter::Acquire() {
  AsyncAcquire().get();
}

I do not want to do this, because in practice the async sleeper will be implemented by a CompletionQueue. I do not want to involve a CompletionQueue in the synchronous code paths.

A follow up PR will add a CompletionQueue as a parameter to the factory function.

---

Take this opportunity to reorder MakeMutateRowsLimiter() to return early from the simpler branch.

---

This change is 

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (4dbcd15) 93.00% compared to head (cde8bef) 92.99%.

Files	Patch %	Lines
...loud/bigtable/internal/mutate_rows_limiter_test.cc	76.47%	8 Missing ⚠️
...gle/cloud/bigtable/internal/mutate_rows_limiter.cc	94.11%	1 Missing ⚠️
...ogle/cloud/bigtable/internal/mutate_rows_limiter.h	50.00%	1 Missing ⚠️
.../cloud/bigtable/testing/mock_mutate_rows_limiter.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13192      +/-   ##
==========================================
- Coverage   93.00%   92.99%   -0.01%     
==========================================
  Files        2138     2138              
  Lines      185991   186024      +33     
==========================================
+ Hits       172978   172999      +21     
- Misses      13013    13025      +12     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.