PersistedQueriesLink migration

[micimize]

I closed #699 because I was merging modularization into beta – reopening here

Then once we've got a beta, I'll merge and publish the request method context in gql-dart/gql#123, and break the dynamic serialization context into a custom serializer provided by graphql for use with apq and merge in #699

Adds a v4 based PersistedQueriesLink leveraging changes in gql-dart/gql#123

While i have a vague sense of what this code does, I mostly just focused on getting the tests to pass. I'm also not sure where exactly this link belongs (in core, as a standalone gql link, or as an independent project. Anyhow, will prob come back and triage more after the gql PR is refined/merged.