Add HTTP Headers options for HTTP Monitors

**Is your feature request related to a problem? Please describe.**
If the url requires any kind of header, like authentication. We can't add it.

**Describe the solution you'd like**
I think an option to add custom headers to the requested URL should do most of the job related.

**Describe alternatives you've considered**
I though a solution similar to what we do on postman.

**Additional context**
No context needed

No files or tests are named in the issue. Start by locating the HTTP Monitor configuration and request execution paths, then trace how monitor options reach the requested URL. Done means a user can configure custom headers for an HTTP Monitor and those headers are sent with the request.

Written by the indexing model from the issue text.