Only configure protobuf gRPC generation for gRPC dependencies

[xfocus3]

Closes gh-50822

Spring Boot's Gradle plugin currently adds the protobuf grpc plugin to every generate proto task whenever the protobuf plugin is applied. In a protobuf-only build that has no gRPC dependencies, this makes generateProto try to resolve io.grpc:protoc-gen-grpc-java:null.

This updates the protobuf plugin reaction so the grpc plugin is only added to generate proto tasks when a gRPC dependency is declared. Protobuf-only builds still get the managed protoc artifact, while gRPC builds keep the existing @generated=omit configuration.

Verification:

• ./gradlew --no-daemon :build-plugin:spring-boot-gradle-plugin:test --tests org.springframework.boot.gradle.plugin.ProtobufPluginActionIntegrationTests
• ./gradlew --no-daemon :build-plugin:spring-boot-gradle-plugin:checkstyleMain :build-plugin:spring-boot-gradle-plugin:checkstyleTest
• git diff --check HEAD^ HEAD

[wilkinsona]

Thanks for the proposal. Resolving a configuration while configuring a task, even lazily, is to be avoided so we can't apply this fix.