feat(oauth): add associatedRefreshToken property to /verify

[eoger]

Prerequisite for https://github.com/mozilla/fxa-auth-server/issues/2547 and builds on top of https://github.com/mozilla/fxa-oauth-server/pull/588. This will allow the fxa-auth-server to figure out which refresh token is associated with an auth token used for authentication, so we can tie refresh tokens to devices records.

[GitCop]

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