codegen/protobuf/scala: wire up protobuf_sources(grpc=True) (feature parity with the Java backend)

[robertpi]

protobuf_sources(grpc=True) currently has no effect when the target is resolved by the Scala codegen backend (pants.backend.codegen.protobuf.scala) — no *Grpc.scala service stubs are generated and the ScalaPB gRPC runtime is never injected as a dependency. The equivalent Java backend (pants.backend.codegen.protobuf.java) already reads this field correctly; this PR mirrors that existing, accepted pattern for Scala.

Two independent pieces, closing the gap end-to-end:

1. rules.py — generate_scala_from_protobuf now reads ProtobufGrpcToggleField off the protocol target and, when set, passes ScalaPB's grpc: colon-modifier via --scala_out=grpc:<dir> (instead of the always-bare --scala_out=<dir>), enabling ScalaPB's built-in gRPC service-stub codegen — the same modifier sbt-protoc/ScalaPB passes automatically whenever a .proto declares a service.
2. dependency_inference.py — ScalaPBRuntimeDependencyInferenceFieldSet now also requires ProtobufGrpcToggleField, and infer_scalapb_runtime_dependency conditionally injects the scalapb-runtime-grpc_<binary_version> artifact plus io.grpc:grpc-stub/grpc-protobuf/grpc-netty-shaded when grpc=True, resolving the correct scala-binary-versioned coordinate the same way the base scalapb-runtime dependency already does. Without this, generated *Grpc.scala files from (1) fail to compile (object grpc is not a member of package scalapb).

No behavior change for existing users who don't set grpc=True on a Scala-resolved protobuf_sources/protobuf_source target — the toggle defaults to False and both code paths are no-ops in that case, exactly as they are today.

Provenance: this fix was originally written and verified against a pinned Pants 2.32.0 install in a downstream monorepo (applied as a direct source patch to the installed distribution, since neither touched @rule is override-able from a plugin — both are the sole concrete rule bound to their respective UnionRule). This PR re-diffs that logic against current main.

Testing: added test_generates_grpc_scala to protobuf/scala/rules_integration_test.py, mirroring the existing protobuf/java/rules_integration_test.py::test_generates_grpc_java and the neighboring test_generates_fs2_grpc_via_jvm_plugin for file-naming conventions.

Closes #23528