Added convenience methods to credentials.

[Randy808]

No description provided.

[nepet]

@Randy808 you may want to rebase this before you put any more work into it, some parts changed, e.g. We reintroduced the fn load_file_or_default(varname: &str, default: &[u8]) -> Result<Vec<u8>> for the CA_CRT and the nobody key/cert pair.

[nepet]

Again! Nice work @Randy808

One thing that I noticed: I think we can get rid of providing the node_id to the scheduler on scheduler::new() altogether. We could add it to the Device credentials e.g. as a trait NodeIdProvider instead of deriving it from the tls_config. The non-authenticated methods register, recover could get the node_id from the signer that they get. This may allow us to completely remove get_node_id_from_tls_config.

What do you think?

Also: We may consider adding with_identity only to the Nobody credentials to avoid any confusion for developers about the use of the Device credentials that should be created from bytes or path.
I am not sure if it would make sense to change the signature of Nobody::with to use an Option for the ca param instead that falls back to `load_file_or_default. Let me know what you think about this.

[cdecker]

I see a couple of discussions still ongoing, shall we draft this until the discussion has settled, or are these things that we should spin out?

[cdecker]

Looks like this is runs into the cln-vm-manager issue that @ErikDeSmedt is addressing in #442. I will rebase once that's merged and merge this asap.

[cdecker]

ACK 400fc43