Support `extend type Filesystem`

[sipsma]

User extensions should be able to add fields to Filesystem (and in general, any other types). E.g. this could enable something like

{
   core {
    git(remote:"github.com/foo/bar") {
      yarn(script:"build") {
         ...
      }
    }
   }
}

Basically, chains become even more powerful.

Based on a quick test this actually appears to work as of today with the exception that codegen for go doesn't generate methods for implementing the filesystem subfield resolvers. But if you manually add the resolver by hand it works; schema is stitched correctly and resolver is routed as expected.

[sipsma]

Ancient issue, decided to not support in modules implementation