GH-24: Add ability to generate gRPC stubs

[ascopes]

Adds ability to generate gRPC stubs for Java and optionally Kotlin by resolving the binaries from Maven central.

The plugin structure has been moved around to enable reusing components more easily.

Closes GH-24.

[codecov]

Codecov Report

Attention: 107 lines in your changes are missing coverage. Please review.

Comparison is base (2ecc8e8) 36.33% compared to head (66844ca) 29.51%.

Files	Patch %	Lines
.../protobufmavenplugin/generate/SourceGenerator.java	0.00%	51 Missing ⚠️
...opes/protobufmavenplugin/AbstractGenerateMojo.java	0.00%	11 Missing ⚠️
...lve/grpc/MavenGrpcJavaPluginCoordinateFactory.java	0.00%	9 Missing ⚠️
...e/grpc/MavenGrpcKotlinPluginCoordinateFactory.java	0.00%	9 Missing ⚠️
...obufmavenplugin/execute/ProtocArgumentBuilder.java	0.00%	8 Missing ⚠️
...ufmavenplugin/generate/SourceGeneratorBuilder.java	0.00%	5 Missing ⚠️
...lugin/resolve/grpc/PathGrpcJavaPluginResolver.java	0.00%	3 Missing ⚠️
...gin/resolve/grpc/PathGrpcKotlinPluginResolver.java	0.00%	3 Missing ⚠️
...opes/protobufmavenplugin/GenerateProtobufMojo.java	0.00%	2 Missing ⚠️
.../protobufmavenplugin/GenerateProtobufTestMojo.java	0.00%	2 Missing ⚠️
... and 2 more

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #25      +/-   ##
============================================
- Coverage     36.33%   29.51%   -6.82%     
  Complexity       64       64              
============================================
  Files            18       24       +6     
  Lines           355      437      +82     
  Branches         23       29       +6     
============================================
  Hits            129      129              
- Misses          225      307      +82     
  Partials          1        1              

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