Allow return type specification in IFieldResolver

[oscarnylander]

• This adds a fourth parameter to IFieldResolver: TReturn
• TReturn defaults to any for backwards compatibility

Relates to #704

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

[apollo-cla]

@hedvigoscar: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[mfix22]

@hedvigoscar are you able to get this PR up to date with master?

[oscarnylander]

@mfix22 Should be good now I think 👍

[mfix22]

@hwillson just wanted to confirm with you before merging since I haven't updated the types before 👍

[ericbiewener]

Any update on this? Seems like a desirable addition.

[ardatan]

Thanks @hedvigoscar !

[oscarnylander]

🙇