feat(bigtable): add support for AsyncPrepareQuery

[diegomarquezp]

Part of the Bigtable GoogleSQL feature effort.

Details:

• Added AsyncPrepareQuery to the generator config textproto

Other necessary RPCs PrepareQuery and ExecuteQuery are already included (see full search):

google-cloud-cpp/google/cloud/bigtable/internal/bigtable_stub.cc

Lines 115 to 116 in 612245c

	StatusOr<google::bigtable::v2::PrepareQueryResponse>
	DefaultBigtableStub::PrepareQuery(

google-cloud-cpp/google/cloud/bigtable/internal/bigtable_stub.cc

Lines 127 to 129 in 612245c

	std::unique_ptr<google::cloud::internal::StreamingReadRpc<
	google::bigtable::v2::ExecuteQueryResponse>>
	DefaultBigtableStub::ExecuteQuery(

---

This change is 

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.04%. Comparing base (612245c) to head (ba98b69).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...loud/bigtable/internal/bigtable_channel_refresh.cc	0.00%	3 Missing ⚠️
google/cloud/bigtable/testing/mock_bigtable_stub.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15457   +/-   ##
=======================================
  Coverage   93.04%   93.04%           
=======================================
  Files        2403     2403           
  Lines      219553   219557    +4     
=======================================
+ Hits       204273   204278    +5     
+ Misses      15280    15279    -1     

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