[RFC] Field extensions

[martinbonnin]

Not being able to add directives to an existing field definition has been a growing pain point in Apollo Kotlin. This RFC solves that.

• The RFC PR is there.
• You can read the formatted RFC there.

This issue is the tracking issue that can be labelled/updated/commented. Feel free to comment here for general feedback or in the PR for more specific ones.

See #952

[egoodwinx]

I'll work on this!