Upgrade Protobuf Dependency

[wozz]

• Upgrades protobuf dependency to be compatible with the new v2 golang protobuf API.
• Switches to go modules dependency tracking, drops vendoring
• Updates WORKSPACE name
• Drops support of gogo due to lack of support for the new API and GoGo Protobuf looking for new ownership gogo/protobuf#691

[akonradi]

Dropping GoGo support seems like it merits discussion. @kyessenov, you requested and added support for GoGo proto, what are your thoughts?

[wozz]

FWIW, go-control-plane no longer uses gogo protos as of envoyproxy/go-control-plane#226

[Mythra]

This seems to also be a lot of changes in one, which is concerning incase we run into problems with just one of the changes that we weren’t expecting, or for the ones that require more discussion like gogo support (I realize go-control-plane doesn’t use it anymore, but do others? Do they need it for a bit longer to migrate off?)

Is there any particular reason we can’t separate these to say:

• one for upgrading to protov2
• one for dropping gogo support
• one for changing the local build to go mod?

[wozz]

sure, I can work on separating the changes, but they will probably need a specific order. I kind of worked backwards on this from my goal of adding proto v2 support.