[8.19](backport #44923) [filebeat][GCS] - Added missing 'text/csv' content-type filter support

[mergify]

Type of change

• Bug

Proposed commit message

Currently content-type "text/csv" did not have official filtering
support even though we support csv decoding. This made objects having this
content type defined were not picked up for processing. This PR as a "bugfix",
adds support for explicit "text/csv" content-type filtering while processing 
each job. 

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes [filebeat][GCS] - Uncompressed CSV's with default content-type 'text/csv' are rejected #44922
• Relates [netskope] Support Netskope Log Streaming integrations#14032

Use cases

Screenshots

Logs

---

This is an automatic backport of pull request #44923 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of a2b74e8 has failed:

On branch mergify/bp/8.19/pr-44923
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit a2b74e866.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG.next.asciidoc
	modified:   x-pack/filebeat/input/gcs/mock/data_files.go
	modified:   x-pack/filebeat/input/gcs/mock/mock.go
	new file:   x-pack/filebeat/input/gcs/mock/testdata/txn1.csv
	modified:   x-pack/filebeat/input/gcs/types.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/filebeat/input/gcs/input_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[ShourieG]

/test