Generate getters for interface fields

[neptoess]

Closes #1469
The Star Wars example has a small hack in it (GetFriendsConnection / GetFriends) due to the custom resolver logic. That may need looked at, since it's no longer a very clear cut example

[neptoess]

I see a lot of the checks failing. Didn’t realize quite how many tests there were throughout the codebase. Are any safe to ignore, or should I go into each and make sure they all pass?

[giautm]

I see a lot of the checks failing. Didn’t realize quite how many tests there were throughout the codebase. Are any safe to ignore, or should I go into each and make sure they all pass?

There is some issue with the template, you need to fix all issues.

template.gotpl: template: template.gotpl:39:17: executing "template.gotpl" at <.Interfaces>: error calling Interfaces: not found

[neptoess]

The lint step is failing because of a generated code diff. No matter how I try to run go generate locally, including running ./.github/workflows/check-generate directly in my local bash, I can't seem to generate anything different. Not sure how to fix this one.

EDIT: In case it's relevant, running Windows 10 64-bit, Go 1.19
EDIT 2: Workflow results https://github.com/neptoess/gqlgen/runs/7695262937?check_suite_focus=true#step:6:27
EDIT 3: Passing now. There must be some small differences in whitespace behavior with text/template introduced in 1.19, so I just manually popped in the offending whitespace

[coveralls]

Coverage decreased (-0.2%) to 74.801% when pulling 0a2f820 on neptoess:add-interface-getters into 0d91c89 on 99designs:master.

[StevenACoffman]

So the generated code comment has either one or two leading spaces depending on Go 1.18 vs Go 1.19:

//  These things are all valid, but without care generate invalid go code

Which originates from the GraphQL schema comment:

""" These things are all valid, but without care generate invalid go code """

Not sure what is different, but that's probably a separate issue from this PR, so it's ok to have manually fudged it here.

[StevenACoffman]

Please see #2331