Add --proto-out-dir option to generate .proto files

[Eitol]

Feature Description:

Add a new option --proto-out-dir to grpcurl that allows users to generate .proto files for listed, described, or invoked elements and their transitive dependencies.

Functionality:

When using the list and describe verbs, the listed or described elements and their transitive dependencies will be written as .proto files in the specified directory.

Proposed Implementation:
Add a new flag:

protoOut = flags.String("proto-out-dir", "", "The name of a directory where the generated .proto files will be written.")

Use Case:
This feature will be useful for users who need to generate .proto files from gRPC services, especially when working with services that don't have readily available .proto files or when dealing with dynamically generated services.

Would you like me to modify or expand on any part of this issue?