Http transport add custom headers

[DungTiger]

Hello,

We use the Http transport and we're trying to move some of our apps behind a reverse proxy.
In order to not have any issues with ingestion, we would need to set some custom headers on the requests so they are not stopped before reaching Graylog.
I checked the code and it's not possible at this point.

I'm willing to do the implementation - is this repo still maintained and accepts pull requests?