Feature Request: Flexible HD Derivation Path & Optional BIP39 Passphrase Support

### What is this about?

Hello MetaMask team,

This is a feature request focused on advanced HD wallet functionality for power users.

1) Account-level HD derivation path flexibility
MetaMask currently derives accounts using a fixed path pattern (m/44'/60'/0'/0/index).
It would be useful to allow advanced users to select or customize the account-level derivation path
(e.g. m/44'/60'/1'/0/* or other BIP44-compatible paths),
to improve interoperability with hardware wallets and advanced wallet setups.

2) Optional BIP39 passphrase support
Adding optional BIP39 passphrase (“25th word”) support would provide an extra security layer
without changing the mnemonic phrase, and align MetaMask with many hardware and advanced software wallets.

Both features could be placed behind advanced settings with clear warnings,
so they do not affect default user experience for beginners.

Thank you for your consideration.

### Scenario

_No response_

### Design

_No response_

### Technical Details

_No response_

### Threat Modeling Framework

_No response_

### Acceptance Criteria

_No response_

### Stakeholder review needed before the work gets merged

- [ ] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

_No response_

The issue does not name implementation files, entry points, tests, or a concrete scenario. Start by mapping the existing HD derivation and wallet import flows, then define the supported path and passphrase behavior, warnings, compatibility requirements, and security review before implementation. Done requires agreed acceptance criteria and coverage for both features.

Written by the indexing model from the issue text.