Add makeFederationSchemaPlugin.

[wtgtybhertgeghgtwtg]

Description

This allows a user to create a version of the plugin that resolves entities according to how they'd like. This should allow users to write a version of the plugin that uses primary keys as fields for the @key directive without running into the concerns mentioned in #1 (comment). The existing version of the plugin, using nodeId, is created this way and exported as the default. It's not too friendly or intuitive, so, if this is an acceptable direction, I would appreciate input on how to make it more usable.

Performance impact

I have not noticed any performance impact. After schema construction, it shouldn't be any slower than it was before.

Security impact

I am not aware of any risks added, though the new way of handling the @key directive may run into something I have not seen.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

[benjie]

I don't have time currently to review this; may I suggest that you maintain a fork and that we link to that fork from the README?