Keyring Controller: Add method to export vault key

[matthiasgeihs]

Explanation

Keyring Controller:

• Add method controller.exportEncryptionKey to export vault key.
• Change method controller.submitEncryptionKey to have an optional salt.
  • If the salt is provided, the controller will check that it is consistent with the locally stored salt.
  • If the salt is not provided, this check is omitted.
  • Before, the salt was mandatory, but it might not be required in the case of unlocking the vault during vault recovery.

This feature is relevant for resolving an audit finding with the seedless onboarding controller.

References

Previously, seedless onboarding was backing up the keyring password to allow for vault recovery after a password change. Now we backup the keyring encryption key.

See the ADR for more details.
This is part of the implementation of option 6.

Changelog

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, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes