Seperate signing key for LNURLauth

**Is your feature request related to a problem? Please describe.**
Currently the signing key depends on the wallet connector, such as the admin key for LNbits, etc.
In rare keys if the admin key gets compromised or reset, it is tedious to update all websites to use the new signing key.

**Describe the solution you'd like**
Separate signing key that can be backed up, for example a 12 word mnemonic

**Describe alternatives you've considered**
None

**Additional context**
None

**Are you working on this? (Yes/No)**
No