GH-10623: Add `GrpcInboundGateway`

[artembilan]

Fixes: https://github.com/spring-projects/spring-integration/issues/10623

• Add spring-integration-grpc module
• Manage respective new io.grpc dependencies
• Add Checkstyle suppression for new Proto-generated classes
• Add GrpcHeaders for convenient API
• Implement GrpcInboundGateway with a proxy for gRPC service methods
• Add TestInProcessConfiguration for a common in-process infrastructure
• Document new feature