feat: Add SLIP39 (Shamir's Secret Sharing) support for coldkey management

Implements issue #407 - SLIP39 wallet creation and recovery

Features:
- New 'wallet new-coldkey-slip39' command to create coldkeys with SLIP39 shares
- New 'wallet recover-coldkey-slip39' command to recover coldkeys from shares
- Configurable M-of-N threshold scheme (default 3-of-5)
- Optional passphrase protection for additional security
- Save shares to files or display on screen
- Interactive share entry for recovery

Security benefits:
- Plaintext seed phrase is NEVER exposed
- Eliminates risk of screen reader and keylogger attacks
- Shares can be distributed to separate custodians
- Threshold scheme allows recovery even if some shares are lost

Files changed:
- bittensor_cli/src/bittensor/slip39.py - Core SLIP39 utilities
- bittensor_cli/cli.py - New CLI commands
- tests/unit_tests/test_cli.py - 11 unit tests for SLIP39
- pyproject.toml - Added shamir-mnemonic>=0.3.0 dependency

Author: MkDev11