Allow to send extra headers in the requests to the external service

Describe the feature you'd like to request

Some providers require extra headers in the APi request.

For example, OpencodeGo requires a x-opencode-session header that identifies a user session so it can cache efficiently. AFAIK this header is now mandatory in this service.

Some other services might require similar headers. It would be nice to be able to set some extra headers in the admin settings.

Describe the solution you'd like

I think we should wait for #431 to get in.

It could be an additional input field in each "connected service" settings. A text are would work. The expected format could be:

header1: value1
header2: value2

Describe alternatives you've considered

The expected format could also be json:

{
  "header1": "value1",
  "header2": "value2"