credentials/alts: avoid SRV and TXT lookups for handshaker service

[townba]

When dialing the handshake service, disable the service config to ignore any provided by the resolver.

When doing a DNS resolver lookup, don't do an SRV record lookup for the metadata service.

This is a similar approach implemented in grpc/grpc-java#9897.

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.92%. Comparing base (66385b2) to head (7d7352a).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7861      +/-   ##
==========================================
+ Coverage   81.84%   81.92%   +0.07%     
==========================================
  Files         374      375       +1     
  Lines       37993    37982      -11     
==========================================
+ Hits        31096    31115      +19     
+ Misses       5598     5567      -31     
- Partials     1299     1300       +1     

Files with missing lines	Coverage Δ
...ntials/alts/internal/handshaker/service/service.go	69.23% <100.00%> (+2.56%)	⬆️
internal/resolver/dns/dns_resolver.go	89.47% <100.00%> (+0.07%)	⬆️

... and 31 files with indirect coverage changes

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[dfawley]

Please add comments in both places explaining why we're doing these things. Otherwise LGTM, thanks.

[townba]

PTAL