Add `hotkeypub` to bittensor-wallet

[basfroman]

For this task opentensor/btcli#513 we need to have hotkeypub logic.

AC:
Wallet has methods and properties:

• hotkeypub (property)
• hotkeypub_file (property)
• get_hotkeypub
• set_hotkeypub
• unlock_hotkeypub
• regenerate_hotkeypub