Is your feature request related to a problem? Please describe.
The optional message is really useful for displaying information that is not captured by a smart card. For example, I use it to display information about my VPN connection.

I run an auxiliary service that acts as a "relay", Homer initiates a GET to this endpoint, the service does a couple of API calls and returns the information in a structure that Homer expects:

As with every other service I run, it is exposed through a reverse proxy that also uses an authentication middleware, enforcing 2FA. Services play nice with this approach, as I have defined the top-level proxy configuration like so:

proxy:
  useCredentials: true

message however does not respect this, nor does it take its own useCredentials. This is understandable as it is meant to simply interact with public external services, but it would be nice to allow more control over the HTTP headers, allowing passing proxy credentials as well.

Describe the solution you'd like
message should allow proxy configs, similar to any other service.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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