chore: add misc/genprotoc/Makefile

[moul]

I forgot to add it during #585.

---

$ git clean -fxd
$ go mod tidy; echo $?
0

Addresses partially #749.

Fixes #749.

[tbruyelle]

After I ran make -C misc/genproto, I have a small diff in tm2/pkg/amino/tests/pb/tests.pb.go and
tm2/pkg/amino/tests/proto3/proto/compat.pb.go, related to the protoc version.

 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
-// 	protoc-gen-go v1.30.0
+// 	protoc-gen-go v1.28.1-devel

Maybe that's something we can fix by using buf instead of protoc, since it's a go tool, we can probably add it to our misc/devdeps so we get a stable version for everyone.

But we can also probably do that later.

[moul]

§I appreciate the suggestion and we may consider implementing it at a later time, maybe after the exploration of the JS client by @zivkovicmilos.

Since this package is solely for testing, the minor difference can be ignored if forgotten during rebasing. Implementing support for buf is not a priority, and its implementation with Amino could be challenging without limiting it to this specific file. It's noteworthy that genproto calls are currently rare