Extend Custom network API to more fields

- Giving the option to add a price feed link on the custom network form (and API), similar to blockexplorer link.
- GIve the option to add gas estimation API for a custom network

Applies both to Custom Network API, and the custom network form (when user adds a network manually)

Locate the Custom Network API implementation and the custom network form, then inspect how the existing blockexplorer link is represented and handled. Add support for price feed links and gas estimation APIs in both paths, and verify that manually added networks and API-created networks retain these fields.

Written by the indexing model from the issue text.