Add ability to import private keys

MyEtherWallet would like to import private keys to Metamask and let them show up as an another account in MetaMask.

Key management snaps should be able to import private keys in a secure way.

This is dependent on Keyrings and Blockkit like UI.

The issue names key management snaps, Keyrings, and Blockkit-like UI, but no files or tests. Start by locating those components and clarifying the dependency boundaries for secure private-key import. Done means imported keys appear as another MetaMask account without compromising key security.

Written by the indexing model from the issue text.