Modularize and add validation

[rvolosatovs]

Summary:

Closes #21
Closes #18
References TheThingsNetwork/lorawan-stack#51

Changes:

• Migrate to https://github.com/lyft/protoc-gen-star
• Use (forked) https://github.com/lyft/protoc-gen-validate via module system of protoc-gen-star
• Remove dep dependency
• Use protoc from environment by default, not Docker image
• Update copyright

Notes for Reviewers:

This PR uses forks of both https://github.com/lyft/protoc-gen-star (it uses features in lyft/protoc-gen-star#51) and https://github.com/lyft/protoc-gen-validate(minor changes there to improve GoGo support and use fieldpaths)

I forked protoc-gen-validate, since only minor changes are necessary to make it work with fieldpaths(TheThingsIndustries/protoc-gen-validate@0e040cd), however we do not need to maintain any validators and we're fully compatible with the upstream plugin.

cc @KrishnaIyer

[htdvisser]

Generally looks fine to me.

This PR does multiple things at the same time. Please try to scope PRs to a single thing in the future.

Some more comments would be welcome too. You'll make the lives your future self, the rest of the team and open source contributors a lot easier.

[htdvisser]

I would keep the suffix

[rvolosatovs]

It's unnecessary complexity.
We're going to make releases with goreleaser if we need them.
This make target is only useful for development and in that case the GOOS and GOARCH are always known by the caller.

[rvolosatovs]

Here's how the generated code looks like for lorawan-stack: rvolosatovs/lorawan-stack@9d36c19
It's a bit rough around the edges, but good enough for MVP
Note, that there still is a little issue with CustomName handling in oneofs, which I will have to fix in https://github.com/TheThingsIndustries/protoc-gen-star before this can be merged. I will add a testcase for that too.

[KrishnaIyer]

Code looks quite good. Also checked the generated validators.

I have a question with the error handling in the code generated by protoc-gen-validate.

			if !_ApplicationIdentifiers_ApplicationID_Pattern.MatchString(m.GetApplicationID()) {
				return ApplicationIdentifiersValidationError{
					field:  "ApplicationID",
					reason: "value does not match regex pattern \"^[a-z0-9](?:[-]?[a-z0-9]){2,}$\"",
				}
			}

Two questions:

1. Does this output the Go generated fieldname and not the proto name (ex: application_id)?
   That is undesirable since we want to return the proto names to the caller.
2. Does it output the full path of the error(ex: end_device_ids.application_ids.application_id)?

[rvolosatovs]

1. Fixed that in https://github.com/TheThingsIndustries/protoc-gen-validate
2. We do get the full path when error occurs: invalid Test.a: embedded message failed validation | caused by: invalid Test_TestNested.a: embedded message failed validation | caused by: invalid Test_TestNested_TestNestedNested.a: value must be inside range (24, 42]
   We may want to use the proto names for types here also, but that's not a priority now - please create an issue at https://github.com/TheThingsIndustries/protoc-gen-validate if this is required