Separate secret key material from config parameters in LSPS2 and LSPS5

[martinsaposnic]

As discussed here #3662 (comment)

Currently, secret key material such as signing_key in LSPS5 and promise_secret in LSPS2 is included directly in the Config structs. While this works for now, it's not ideal to mix sensitive cryptographic material with general configuration parameters.

We should consider refactoring the codebase to store secrets separately from the rest of the configuration data.