Allow configuring headers in outbound push

[witash]

Outbound push may sometimes require different HTTP headers such as User-Agent; currently its not possible to add or change HTTP headers other than authorization.

Allow headers to be added or changed in configuration like

        "headers": {
          "User-Agent": "CHT-Integration/1.0 libwww/2.17b3"
        },

[gitcliff]

Format https://httpwg.org/specs/rfc9110.html#field.user-agent

[mrjones-plip]

@witash - I think not setting a user agent is bug (per the forum that inspired this ticket).

Maybe more tightly scope this ticket to just be:

Add user-agent header to outbound push

I can't see a need for additional headers just yet - do you know of a need?