feat: add opt_file_args to pass the file arguments to the protoc plugin

[viacheslav-rostovtsev]

Currently opt_args parameter allows passing the string arguments to the protoc plugin via the -<plugin>_opt= protoc command line argument. This does not allow passing the arguments that point to the files.
This PR introduces the new parameter to the proto_custom_library rule: opt_file_args
It works similarly to the plugin_file_args to allow passing the file-arguments to the protoc plugins.

[codecov]

Codecov Report

Merging #3239 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3239      +/-   ##
============================================
- Coverage     87.12%   87.12%   -0.01%     
+ Complexity     6077     6076       -1     
============================================
  Files           494      494              
  Lines         24052    24052              
  Branches       2613     2613              
============================================
- Hits          20956    20955       -1     
  Misses         2234     2234              
- Partials        862      863       +1     

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/google/api/codegen/discovery/Schema.java	84.34% <0.00%> (-0.51%)	42.00% <0.00%> (-1.00%)

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[alexander-fenster]

@viacheslav-rostovtsev just FYI, I renamed the PR to match the conventional commit standards which is needed for a release bot. (I'll leave it to @vam-google to review)

[vam-google]

LGTM, but please verify that it works on googleapis first (this rule is used absolutely everywherfe for the build, so changes in it must be made carefully