[feature]: Import a private key

[eshon]

Summary

Add the ability to import a Filecoin private key into the FilSnap (as available in the MetaMask GUI) by using either:

1. a private key string
2. a JSON file upload

Expected behavior / User Story

Completion

When I want to import a private key I am given the option of importing a private key string or uploading a JSON file containing the private key.

When I import the key using either method, the FilSnap loads my Filecoin account into the FilSnap UI as another account I can use.

(Optional) I can switch between recently imported and created accounts as in the MetaMask UI.

[eshon]

Sounds like this is currently blocked by the MetaMask SNAP API and needs discussion with the MM team