[fix] allow renaming of subscription root fields

[yaacovCR]

Fixes #997.

• NA: 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.

[yaacovCR]

New test added for this functionality passes.
Other tests fail because they are failing on master anyway, fixed separately in #1102.

[yaacovCR]

In addition to allowing explicit renaming of subscription fields, this fix should allow delegation of a subscription root field to another schema's subscription root field regardless of name. @benjamn, @hwillson, any thoughts?