Add an API option to import a seed phrase

[westonb7]

What problem are you trying to solve?
We're looking for ways to integrate SeedQuest (https://github.com/reputage/seedQuest) with MetaMask, and after exploring the options available through plugins, determined that a plugin wouldn't quite work for what we're trying to accomplish.

Describe the solution you'd like
We want to allow MetaMask users to import a seed phrase from an external Dapp or website, which could be used to allow the user to access accounts/keyrings in MetaMask by importing seed phrases from external sources.

Additional context
We (the SeedQuest team) would be happy to collaborate and help out in any way we can.

[rickycodes]

closing because dupe of #7364