feat(internal/gensnippets): initial commit (#3922)

This tool renders all examples in the current directory to standalone
files.

I confirmed this successfully processes every example in this repo. Once
I added a go.mod/go.sum in the generated directory, all of the examples
passed go vet.

A future change will integrate this tool into our automatic generation
pipelines and add region tags.

Author: tbpg

internal/gensnippets/go.mod

@@ -0,0 +1,12 @@
+module cloud.google.com/go/internal/gensnippets
+
+go 1.16
+
+replace cloud.google.com/go/internal/godocfx => ../godocfx
+
+replace cloud.google.com/go => ../..
+
+require (
+	cloud.google.com/go v0.81.0
+	cloud.google.com/go/internal/godocfx v0.0.0-00010101000000-000000000000
+)