add option to export seed phrase or private key for hot signers

[MatheoGrivault]

This PR adds an option to export the seed phrase or one of the private keys of a hot signer (the password must be entered before exporting).

Exporting the seed phrase (mnemonic) for a previously added signer isn't possible since only the seed is saved on disk but new signers now also save the encrypted seed phrase.

[floating]

Hey @MatheoGrivault, just wanted to drop by to say thanks for the PR! We're planning to loop back on this for an upcoming v0.7.x release.