⚗️Key derivation and secret seed storage

[LLFourn]

The purpose of this spike is answer the following:

How should we take some secret data and use it to derive:

• transient keys within the various protocols
• the node identifying key

How should we store this secret?
How should the generation happen? Automated or Manual?
Should we provide backup strategies (ie, mnemonic)

hint: look at other projects an summarise the best approaches

Child of #744

[thomaseizinger]

Cross referencing an idea here on how to extend the Seed type:

#878 (comment)