Bin_node: create separate lib for node config

Context

In order to reuse it elsewhere (so far 2 instances : generate some doc and make the proxy server use it), we want the logic definition of a node config to be a lib.

This is extracted out from !6555 (closed).

Manually testing the MR

This MR mustn't have any visible consequences. Pure reorg!

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR