Backporting PR #818 to v0.38.x-experimental branch

Commit to backport: 65a88b9 #1437

Branch v0.38.x contains a legacy gRPC implementation.

Branch main does not have the legacy gRPC, which has been removed, and #818 adds a new gRPC service.

When backporting #818 we have two options:

1. Remove the legacy gRPC and replace it by the new gRPC service implemented in main
   • For preserving the existing functionalities in v0.38, this approach requires implementing the existing two legacy gRPC endpoints (BroadcastTx and Ping) using the new gRPC service
2. Preserve the legacy gRPC implementation and add the new gRPC implemented in main as an additional service
   • In this case, 3 RPC endpoints could be available on a node: the standard JSON-RPC, the legacy gRPC, and the newly introduced gRPC service

Additional changes:

• Update tests to ensure that the legacy gRPC endpoints works as before
• Update e2e test infra to: in case of approach 1, add test for the legacy endpoint to ensure it works with the API; in case of approach 2, extend the e2e node to support 3 endpoints
• Changelog entries explaining what is breaking compared to v0.38.x, if any. (there were none)