Extract GenerateProtoTask querying fields to ProtoVariant (2/3)

[rougsig]

Background
To prevent eager task configuration for android, we need to hide generateProtoTask from user.

Changes
Extract task variant information (sourceSetName, buildTypeName, flavorName, etc) to ProtoVariant class.
Use ProtoVariant as tags to select GenerateProtoTaskSpec for configuration.

Breaking Changes
generateProtoTask {} api have breaking changes.

// before
generateProtoTask {
  all().each { GenerateProtoTask task ->
    task.plugins {
      grpc { }
    }
    // other configuration logic
  }
}

// after
generateProtoTask {
  all { // GenerateProtoTaskSpec spec -> (implicit this, no direct access to GenerateProtoTask instance)
    plugins {
      grpc { }
    }
    // other configuration logic
  }

  // ofSourceSet, ofTest, etc have the same changes
}

Test plan
Green pipelines.

Note
Merge after #668

[rougsig]

note:
second small part for fix for #643.

Third part will be use changes from 1, 2 PR, to configure agp/other tasks lazily.