feat: optionally limit access to nodes listed in authorized_keys file

[Frando]

Allows to limit access to a dumbpipe server to nodes listed in an authorized_keys file.

Quoting from the new section in the README:

You can limit access to a dumbpipe listener through a keys file, similar to the authorized_keys file that SSH uses.
You can put the file wherever you want, e.g. at ~/.dumbpipe/authorized_keys. For the file to be used, and thus
access to be limited, specify the file path with the --authorized-keys (or -a) when launching dumbpipe.
When authorization is set, only connections from nodes listed in the file will be accepted.

Here's an example file:

# dumbpipe authorized nodes
148449487b53bb90382927634114457ef90d2a63127200fd8816a8dffb9d48c6 some-server
3827f5124d03d10f2f344d319a88c64c198c4db1335560ea6aad41ce2fb7c311 devbox

The file must contain a list of hex-encoded node ids, seperated by newlines.
The node ids may be followed by a comment, separated by a space from the encoded node id.
Lines starting with # are ignored and can be used as comments.

[matheus23]

It might be reasonable to check for the 403 error code here to provide a nice error message when you're not authorized. Perhaps saying something like "The remote node denied access: Your node ID is not configured as authorized in the remote authorized_keys file."

[maan2003]

implemented based on cli flags, it is less state. maan2003@f22fc7e

[rklaehn]

Given that dumbpipe is supposed to be a very simple and lightweight tool, I am in favour of using just CLI args for this. Adding a file with an associated file format feels a bit heavy.