Add <alertmanager_config>.http_headers configuration

[giovanniuum]

In the most recent version (2.55) of Prometheus, there is a http_headers field allowed in each alertmanager_config block.
In some cases, the alertmanager instance is behind a gateway that needs to check the Prometheus signature using custom headers.
With current Promxy's version of Prometheus, this feature is not available.
Maybe let's upgrade Prometheus engine version to last version (2.55)?