chore: re-work Bazel build to accommodate Maven setup

[chanseokoh]

Changes:

1. Moving src/test/resources/*.proto into src/test/proto/.
   This makes it natural to use protobuf-maven-plugin in a future Maven setup.

2. Embedding service_config.proto into src/test/proto/.
   Instead of having Bazel pull the grpc-proto repo to reference service_config.proto there, this PR keeps a copy. The repo README says

   Non-Bazel users are expected to copy the proto files from this repo as needed.

   Optionally in the future, Maven could be configured to dynamically download the file, but I'm not sure if it's really worth it.

3. Changes to Bazel *.java compilation setup
   Looks like Bazel is not so smart as I had hoped, and I think it is not efficient unless we compile *.java and declare it as a separate library first and then use it in java_binary() or java_test().

4. Changes to how resource files are accessed in tests
   This is to make both Bazel and Maven tests work in harmony in the future.

[codecov]

Codecov Report

Merging #936 (da19e0e) into main (2a3df94) will increase coverage by 3.75%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##             main     #936      +/-   ##
==========================================
+ Coverage   87.37%   91.13%   +3.75%     
==========================================
  Files         162      265     +103     
  Lines       16307    27632   +11325     
  Branches     1179     1203      +24     
==========================================
+ Hits        14248    25182   +10934     
- Misses       1717     2096     +379     
- Partials      342      354      +12     

Impacted Files	Coverage Δ
...com/google/api/generator/test/framework/Utils.java	30.00% <50.00%> (ø)
...or/gapic/protoparser/SourceCodeInfoParserTest.java	91.13% <56.25%> (ø)
...le/api/generator/engine/JavaCodeGeneratorTest.java	100.00% <0.00%> (ø)
...ogle/api/generator/engine/lexicon/KeywordTest.java	100.00% <0.00%> (ø)
.../generator/gapic/model/GapicServiceConfigTest.java	98.76% <0.00%> (ø)
...m/google/api/generator/gapic/model/MethodTest.java	100.00% <0.00%> (ø)
...mposer/samplecode/SampleCodeJavaFormatterTest.java	91.30% <0.00%> (ø)
...rator/gapic/protoparser/ServiceYamlParserTest.java	100.00% <0.00%> (ø)
...ator/gapic/protoparser/ResourceNameParserTest.java	98.64% <0.00%> (ø)
...gle/api/generator/engine/lexicon/OperatorTest.java	100.00% <0.00%> (ø)
... and 95 more

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