[Feature request] Allow to override main outbound invite headers

[devniel]

Hello,

Please, Is it possible to add a way to override main headers?, that way we can set custom From headers which are different for some telephony providers

 sip.CreateSIPParticipantRequest(
  sip_trunk_id=sip_trunk_id,
  sip_call_to="33749127211",
  sip_number="2505272605",
  headers={
   "From": "Anonymous <sip:[email protected]>;tag=cbfbeb02-e408-4dd4-8274-eb87d39211f1"
 },
)

With the current implementation, the headers are appended and then a 'Bad Request' is thrown because 2 From headers were sent.

https://github.com/livekit/sip/blob/main/pkg/sip/outbound.go#L872

I will open a PR if possible as the change is simple.

Thanks!