Add Support for Custom Headers on Custom Networks

**What problem are you trying to solve?**
I'd like to be able to connect to a node behind an API Gateway using an `x-api-key` header.

**Describe the solution you'd like**
I would like to see support for custom headers when creating a custom network connection.

Start by locating the code that creates custom network connections and checking how connection options are passed to the node. The work is done when custom headers such as x-api-key can be configured for a custom network and are included in requests through the API Gateway path; the issue names no files or tests to run.

Written by the indexing model from the issue text.