Merge the extension ENS controller with the core ENS controller

[Gudahtt]

MetaMask mobile and extension both have ENS controllers, but they have separate implementations and even separate purposes.

The mobile ENS controller (here in the core repository) handles ENS lookups and caches the responses. Extension does not use a controller for this; the lookups are performed in the UI and not cached.

The extension ENS controller performs reverse ENS lookups, resolving an address to an ENS name. Mobile doesn't seem to have this feature, though I'm not certain.

Both projects would benefit from both features. We can merge the two into a shared ENS controller that supports forward and reverse ENS lookups with caching for both.

Definition of Done

• Whenever applies, any change unit tested, reviewed(approved) and documented(JSDOC at least)
• We have changelog entries for any related changes
• Any changed APIs have comprehensive inline documentation
• Any changed public APIs are well covered by unit tests
• Other items
• If a planning/research ticket, then the plan has been reviewed and approved by at least 1 team members

[xinnanyemm]

Hey team! Please add your planning poker estimate with Zenhub @cryptodev-2s @Gudahtt @mikesposito

[legobeat]

Relevant when considering the future development of this controller: MetaMask/specifications#11