Allow user-provided protoc plugins in Bazel

[qdii]

go_proto_library takes compilers as an argument, by which a user can decide the protoc plugins should be used against the protobuf.

In the current version, pgv_go_proto_library hardcodes @io_bazel_rules_go//proto:go_proto on top of its own validation-generating plugin.

However, this plugin doesn't generate source code for services present in the protobuf, but other plugins do.

To give the user some flexibility, please allow protoc plugins to be supplied be the user.

[stale]

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