refactor(bigtable): duplicate AsyncRetryBulkApply

[dbolduc]

Lay the ground work for #9175 . This is the same dance as we did in #9221 .

This PR copies the async_bulk_apply* files as new files with a legacy_ prefix. I will modify the async_bulk_apply* files in a subsequent PR to implement the retry loop helper in terms of the modern API (Stubs, common policies). The legacy_ files will exist for nearly a year until the deprecation policy expires on #8800.

It is broken into 2 commits. The reviewer can look at the first commit only to verify that the legacy_ files have no meaningful changes to them. The reviewer can look at the overall diff to verify that async_bulk_apply* have not changed at all. Also, we remove the async_bulk_apply* files from the builds to "prove" that they are not in use.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #9244 (12b0182) into main (f29a2bb) will decrease coverage by 0.00%.
The diff coverage is 98.43%.

@@            Coverage Diff             @@
##             main    #9244      +/-   ##
==========================================
- Coverage   94.56%   94.56%   -0.01%     
==========================================
  Files        1457     1457              
  Lines      131699   131699              
==========================================
- Hits       124539   124538       -1     
- Misses       7160     7161       +1     

Impacted Files	Coverage Δ
google/cloud/bigtable/table.cc	99.27% <ø> (ø)
.../bigtable/internal/legacy_async_bulk_apply_test.cc	98.13% <98.13%> (ø)
...cloud/bigtable/internal/legacy_async_bulk_apply.cc	100.00% <100.00%> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	97.75% <0.00%> (-0.25%)	⬇️
google/cloud/bigtable/internal/async_bulk_apply.cc
...e/cloud/bigtable/internal/async_bulk_apply_test.cc

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