Add C-Lightning hsm_secret backup

BTCPay has a backup UI for LND's Static Channel Backup (SCB), which is basically the private key of the on-chain wallet. The C-Lightning equivalent would be backing up the hsm_secret key. This is relatively easy to do:
https://lightning.readthedocs.io/BACKUP.html

There are two possible options:
1. Allow the user to save the hex dump
2. C-Lightning has the ability to generate the hsm_secret from BIP39 words, although conversion from hsm_secret to BIP39 seems to be missing. Still, you could generate a wallet, show the user the BIP39 words to save them, and use lightning-hsmtool to generate the hsm_secret.

NOTE: A robust backup for the C-Lightning database would still be needed, such as is suggested in issue #393.

Start with the C-Lightning BACKUP documentation linked in the issue and inspect BTCPay's existing LND Static Channel Backup UI for the relevant entry point. Compare the hex-dump and BIP39 options, then define a backup flow that lets users preserve the hsm_secret; verify that the separate C-Lightning database backup concern from issue #393 remains outside this change.

Written by the indexing model from the issue text.