support to generate model for intermediate interface

[Code-Hex]

Describe your PR and link to any relevant issues.

Closes #1690
I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

I hope "small change, small release". Thanks.

ref: vektah/gqlparser#180

[coveralls]

Coverage increased (+0.004%) to 74.295% when pulling 818cbea on Code-Hex:add/gen-intermediate-interfaces into 9546de2 on 99designs:master.

[Code-Hex]

I will update the version of the module when only this change is released
vektah/gqlparser#169

[StevenACoffman]

Can you look into the test failures?

[Code-Hex]

@StevenACoffman this failure caused by gomod. (currently used master branch)

[Code-Hex]

But what happened...

.github/workflows/check-fmt
  .github/workflows/check-fmt
  shell: /usr/bin/bash -e {0}
  env:
    GOROOT: /opt/hostedtoolcache/go/1.16.14/x64
go: github.com/99designs/gqlgen@v0.0.0-00010101000000-000000000000 requires
	github.com/vektah/gqlparser/v[2](https://github.com/99designs/gqlgen/runs/5282597021?check_suite_focus=true#step:5:2)@v2.[3](https://github.com/99designs/gqlgen/runs/5282597021?check_suite_focus=true#step:5:3).2-0.20220221192817-3e788[6](https://github.com/99designs/gqlgen/runs/5282597021?check_suite_focus=true#step:5:6)dd3df0: missing go.sum entry; to add it:
	go mod download github.com/vektah/gqlparser/v2

[Code-Hex]

Probably fixed

[Code-Hex]

I need to fix the test for windows... 🙇

[Code-Hex]

@StevenACoffman @vektah I have done.
I recommend releasing the gqlparser just contains interface implements before merging this PR. and update gqlparser to the version in this PR. I hope "small change, small release". Thanks

[StevenACoffman]

Thank you for your work, but there's still some lingering work to be done in vektah/gqlparser#181 before we can make a release there.

[StevenACoffman]

Rebased and force pushed, so looks good to me.