[Python] [Feature Request] Allow protobuf and grpcio dependencies to be overridden

[randymaldonado]

Description

From the example provided, a developer will typically construct a BUILD file as follows:

load("@rules_proto_grpc_python//:defs.bzl", "python_grpc_library")

python_grpc_library(
    name = "thing_python_grpc",
    protos = ["@rules_proto_grpc_example_protos//:thing_proto"],
)

However, sometimes, a developer wants to use a specific version of protobuf or grpcio that their project is using. Their BUILD file should look like:

load("@my_projects_pip_deps//:requirements.bzl", "requirement") 
load("@rules_proto_grpc_python//:defs.bzl", "python_grpc_library")

python_grpc_library(
    name = "thing_python_grpc",
    protos = ["@rules_proto_grpc_example_protos//:thing_proto"],
    deps = [
        requirement("grpcio"),
        requirement("protobuf"),
    ],
)

This however does not work because in the current code, protobuf and grpcio python packages are hardcoded to use the versions in rules_proto_grpc_python/requirements.txt

While this one example occurs in the python_grpc_library, similar behavior occurs in python_proto_libraryand python_grpclib_library.

A simple solution might be to just check user provided deps for protobuf or grpcio and to assign a default if none were provided.