[8.19](backport #45088) [filebeat][GCS] - Relax content-type restrictions along with some basic cleanup and refactor (#47966)

Sometimes we require to specify the content-type/encoding of objects due to
limitations on the users end in accessing the remote storage. This PR adds
a generic way to validate config structs for container level overrides and
introduces support for specifying custom content-types and encodings.

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Co-authored-by: Shourie Ganguly <shourie.ganguly@elastic.co>
(cherry picked from commit 5ccf2d9)

# Conflicts:
#	CHANGELOG.next.asciidoc
#	docs/reference/filebeat/filebeat-input-gcs.md
#	x-pack/filebeat/input/gcs/config.go

Author: mergify[bot]