[fileconsumer] Find better approach to identify gzip files

[khushijain21]

Component(s)

Fileconsumer

Describe the issue you're reporting

Currently in filesconsumer, gzipped files are identified by its extension ".gz". We need to find a better solution to identify such files because it is possible to have compressed files without such explicit extension names,

[github-actions]

Pinging code owners for pkg/stanza/fileconsumer: @djaglowski @andrzej-stencel. See Adding Labels via Comments if you do not have permissions to add labels yourself. For example, comment '/label priority:p2 -needs-triaged' to set the priority and remove the needs-triaged label.

[andrzej-stencel]

Perhaps looking at the file's header is an option https://datatracker.ietf.org/doc/html/rfc1952#section-2.3.1.

2.3.1. Member header and trailer

     ID1 (IDentification 1)
     ID2 (IDentification 2)
        These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139
        (0x8b, \213), to identify the file as being in gzip format.