add support for @latest tag name for generators

[pkieltyka]

with commands like,

//go:generate go run github.com/webrpc/webrpc/cmd/webrpc-gen -schema=databeat.ridl -target=golang -pkg=proto -server -client -out=./databeat.gen.go

the -target=golang will use the master head of the gen-golang templates. At time of writing, this would be v0.12.x-dev

I'm proposing to add -target=golang@latest which would use the latest tagged version. At time of writing, the latest tagged version for gen-golang is v0.11.0

[VojtechVitek]

I agree with the proposal.

On top of that, I think we should make -target=golang more stable. Perhaps, as in Go, it would also download the latest tagged version.

-target=golang # latest tag
-target=golang@latest # latest tag
-target=golang@v0.11.0 # specific tag
-target=golang@master # master branch
-target=golang@some_other_branch # PR branch

Or even better, I propose we //go:embed the latest stable github.com/webrpc/gen-* generators into webrpc-gen binary. This way we could guarantee the stability and interoperability of the default targets.

-target=golang # stable and tested tag embedded into webrpc-gen, ie. v0.11.2
-target=golang@latest # fetch latest tag
-target=golang@v0.11.0 # fetch specific tag
-target=golang@master # fetch master branch
-target=golang@some_other_branch # fetch PR branch

for non-github.com/webrpc/gen-* repositories, the default would be the same as @latest tag

[VojtechVitek]

As of #266, the -target=golang now uses the //go:embeded version of the golang template, pinned at specific version in go.mod file:

webrpc/go.mod

Line 18 in a3fb4fd

github.com/webrpc/gen-golang v0.14.2