Add missing field validations

[rvolosatovs]

Summary:

Currently there is a bunch of fields in our API, which lack validation options.
E.g.

lorawan-stack/api/end_device.proto

Lines 144 to 147 in 4f5dc70

	// LoRaWAN MAC version.
	MACVersion lorawan_version = 2 [(gogoproto.customname) = "LoRaWANVersion"];
	// LoRaWAN PHY version.
	PHYVersion lorawan_phy_version = 3 [(gogoproto.customname) = "LoRaWANPHYVersion"];

Refs #51 #69

Why do we need this?

To make sure we don't store garbage

What is already there? What do you see now?

Possible garbage in fields

What is missing? What do you want to see?

Validation

How do you propose to implement this?

Add the validators.
For the exact case linked I propose (validate.rules).enum.defined_only = true
and (validate.rules).enum.not_in = [0]
See https://github.com/lyft/protoc-gen-validate#enums

[rvolosatovs]

Blocked by #194

[rvolosatovs]

I added a bunch of missing validations in #194, however those were mostly NS or JS-related and obvious ones, like email.
Let's go component-by-component and ensure that all the fields that need validation are validated.
NS and JS should be fine, however we should still look through GS, AS and IS. I'll assign package maintainers.
@johanstokking please check AS and GS-related messages
@htdvisser please check IS-related messages

[htdvisser]

Unblocked since #194 was merged

[johanstokking]

@rvolosatovs checked both, only addition necessary seems to be in #297