clnrest: configurable content-type header

[ShahanaFarooqui]

Make the content-type of REST responses configurable via config options, so that applications can consume the response without writing wrappers/interceptors; if they do not consume default application/json type.

[gudnuf]

I am working on adding properties http_path and http_method to a plugin command's manifest entry, this will allow plugins to set specific REST mappings for their RPCs rather than being fixed at /v1/<command_name>. Seems like a good place to include the content-type of the response, then this data can be handled by CLNRest to create dynamic routes

[BitcoinJiuJitsu]

Open CLN bounty on this:
https://community.corelightning.org/c/cln-bounties/7164-clnrest-configurable-content-type-header