feat(bigtable): support GOOGLE_CLOUD_ENABLE_DIRECT_PATH

[dbolduc]

Fixes #4382 and basically undoes #6175

I set up a VM with the magic subnet. If I set the endpoint as such and make some request, I see that my gRPC peer is 2001:4860:8040:...etc.....

Note that google-c2p:///bigtable.googleapis.com requires the project to be allow-listed.

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This change is 

[google-cloud-cpp-bot]

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For commit: 723f3e3ec088ab135b4a4a16c8d01e199ff4490a

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[codecov]

Codecov Report

Base: 94.21% // Head: 94.21% // No change to project coverage 👍

Coverage data is based on head (883e249) compared to base (883e249).
Patch has no changes to coverable lines.

❗ Current head 883e249 differs from pull request most recent head 723f3e3. Consider uploading reports for the commit 723f3e3 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9978   +/-   ##
=======================================
  Coverage   94.21%   94.21%           
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  Files        1500     1500           
  Lines      140570   140570           
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  Hits       132434   132434           
  Misses       8136     8136           

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