feat: Allow configuration of additional headers

[mterhar]

Which problem is this PR solving?

If running Refinery in an environment that requires mTLS, we need to be able to configure a header to let the proxy know where to pass the traffic along.

Short description of the changes

Adds a configuration that injects headers into each of the signals that comes out of Refinery that are destined for Honeycomb.

1. Telemetry data flowing through the sampling proxy has it added to the DirectTransport http client
2. Logs from the Honeycomb logger have it added to the libhoney-go as a Round Tripper
3. Metrics coming from the OpenTelemetry metrics emitter have them added to the exporter
4. Experimental internal tracing has them added as well, just for troubleshooting if it's ever necessary

[mterhar]

Had to make a tweak to the markdown/yaml generator that uses the configMeta.yaml file. The example I made has 2 colons and since we don't use colon+space to delimit like yaml does, I had to make a splitter function that just looks for the first delimiter and stops trying to split afterwards.