Allow custom derivation path for custom nodes

**What problem are you trying to solve?**
Allow to set a custom derivation path for custom nodes. It will allow to have a better interaction with others chains and reduce risk.

**Describe the solution you'd like**
Add a new field in custom node configuration to allow to set a specific derivation path.

No files, tests, or entry points are named in the issue. Start by locating custom node configuration and the existing derivation-path handling, then determine how a per-node path should be validated and applied. Done means custom nodes can specify and use a specific derivation path without breaking existing configurations.

Written by the indexing model from the issue text.