feat: add KeyringController:accountAdded event + use keyring events in AccountsController

[ccharly]

Explanation

Moving the add/remove account/address logic in the KeyringController. This way, we don't need to run a complex logic in the AccountsController everytime a new account gets added.

Part of this logic is now at the KeyringController-level yes, but the logic itself seems simpler, and hopefully, is faster too!

References

N/A

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them