Add an API option to export a seed phrase

**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 export their seed phrase directly to an external Dapp or website, which could be used to allow the user to rehearse their seed phrase recovery in SeedQuest.

**Additional context**
Exporting a seed phrase to SeedQuest would ideally be an optional choice that would happen during the onboarding process. This would require an informing prompt, and would be a source of user friction.

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

Start by reviewing the existing plugin and API options described in the issue, then examine how onboarding prompts are handled in the MetaMask extension. Define the security and consent requirements for exposing a seed phrase to an external Dapp, and confirm the design and acceptance criteria with maintainers before implementation.

Written by the indexing model from the issue text.