Feature Request: API to import private key

**What problem are you trying to solve?**
It would be great if a user can import a private key with 1 click from a DApp

**Describe the solution you'd like**
Dapp owner generates a key and has a button to import that key into a metamask
Metamask asks a user if he wants to import a private key

No files, tests, or entry points are named in the issue. Start by reviewing the proposed DApp-to-MetaMask private-key import flow and its user confirmation requirements; done would require an agreed API and implementation scope that safely supports the described interaction.

Written by the indexing model from the issue text.