Add addNewAccountForKeyring method

[mikesposito]

Explanation

This PR adds the addNewAccountForKeyring method to KeyringController, that can be used to add a new account for a specific keyring, which might not be the primary simple keyrings.

To align it with the addNewAccount method that works for primaryKeyring, also addNewAccountForKeyring expects an optional accountCount parameter to make the function idempotent.

This method is needed to migrate some controllers from @metamask/eth-keyring-controller to @metamask/keyring-controller.

References

• Related to Add missing methods to KeyringController #1595

Changelog

@metamask/keyring-controller

• ADDED: Added addNewAccountForKeyring method

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 highlighted breaking changes using the "BREAKING" category above as appropriate

[Gudahtt]

Nit: Typically we separate the test into sections separated by newlines (the sections being "arrange, act, assert" for the setup, code-under-test, and assertions respectively). These tests would be a bit easier to read if we had those newlines surrounding each of these calls under test

[Gudahtt]

LGTM!