fix(resnames): Use anon resname classes when only wildcards are present [ggj]

[miraleung]

Remerge of #761 (already approved) into main.

Use an anonymous ResourceName class as a default value when only wildcard resource name types are present. This primarily affects GCS and IAM. These anonymous classes will be used only for explicit field assignments, and not when the value is being set on a message. Specifically, note that the IAM integration goldens are unchanged.

The GCS use case is covered by the default value unit test. Integration tests may be added once the GCS protos are published.

[codecov]

Codecov Report

Merging #763 (1041962) into master (02eab0e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #763   +/-   ##
=======================================
  Coverage   91.71%   91.71%           
=======================================
  Files         140      140           
  Lines       14795    14795           
  Branches     1052     1052           
=======================================
  Hits        13569    13569           
  Misses        942      942           
  Partials      284      284           

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