Allow for complete Durable Sync engine testing against staging

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This is a Q1 2020 Goal for the SE Team.

Linking all related issues to the efforts to more fully test Sync against staging for all Sync clients.

Related issues:

• [x] move existing sync setup docs to dochub.

Missing functionality:

• [x] Fenix support. Tracking epic for this work is here

• [ ] Lockwise Android Support - won't make it into Q1 for lockwise team.

• [ ] Lockwise iOS Support - won't make it into Q1 for lockwise team.

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Step one: either get a PR open by 2/18 EOD, or need to handoff this one.

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https://github.com/mozilla-mobile/android-components/pull/5984

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Update: handed off the Fenix bits to @eoger (thank you, Ed!) - this work is now tracked here for Fenix.