fix: properly inform cloud endpoints about the snappy compression

[simskij]

Issue

When using Grafana Agent with a logging backend other than Loki, we need to inform that backend about the payloads being compressed using snappy. Loki knows this is the case implicitly, but a third-party won't.

Solution

Add a Content-Encoding header to all outgoing requests.

Context

This is useful when integrating with, for instance, Vector or Logstash, as they otherwise have no way of knowing about the encoding/compression.

Testing Instructions

Release Notes

Properly advertise snappy compression when using the cloud integrator