Create a go module for http2 interop test, fixing breakage from upgrade to go v1.16 (#25707)

Go v1.16 introduced module-aware commands by default. This version
upgrade broke our http2 interop tests (see #25660). Setting the go
environment variable `GO111MODULE` to "auto" makes go tools behave as
they did before the v1.16 change, but will not be compatible with v1.17.
This PR introduces a minimal go.mod file for the interop test to appease go
v1.16.

Docs on the flag: https://golang.org/ref/mod#mod-commands
Recent discussion on golang/go:
  https://github.com/golang/go/issues/31997#issuecomment-782864390

tools/http2_interop/go.mod

@@ -0,0 +1,3 @@
+module github.com/grpc/grpc/tools/http2_interop
+
+go 1.16