Allow to specify `--protocopt` on per-target basis

[auzhva]

Problem Statement

In our project, we prefer to enable strict protobuf validations, with --fatal_warnings on for our targets.

Currently, we're not able to do so if any of the third-party libraries have protobuf files with warnings. So we're looking for some logic that would allow us to:

1. Specify --fatal_warnings for our targets
2. Leave it disabled for third-parties

Proposed Solution

If we take a look into how CC rules are implemented, there are 2 ways to supply CC options:

1. --copt global flag that sets an option for everybody
2. copts attribute of cc_library and cc_binary rules

For protoc, there is --protocopt which is sort of similar to --copt, but there is no way to specify options on a per-target basis. It would be great to see protocopts for the proto_library rule.

Something like:

proto_library(
    name = "sample_proto",
    srcs = ["sample.proto"],
    protocopts = ["--fatal_warnings"], # Here is the added part
    visibility = ["//visibility:public"],
)