[Google Cloud Storage] Add support for alternative_host for system tests

[P1llus]

What does this PR do?

This adds the alternative_host option, which is a field only used for internal testing. By default the input only reaches out to the public cloud GCS API, and this allows us to override this and instead point it to our own emulator running locally for system tests.

Why is it important?

Required for adding system tests for elastic-agent integration packages.

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.

Author's Checklist

How to test this PR locally

1. Start up the local GCS emulator: docker run -p 4443:4443 fsouza/fake-gcs-server -host="0.0.0.0" -public-host=localhost -port=4443 -scheme=http
2. Download stream and run go build : https://github.com/elastic/stream
3. Create a local file for testing, for example test.json, add a JSON object on a single line, like { "testmessage" : "success" }
4. Populate the emulator with the testdata using stream: ./stream log --retry=30 --addr=http://localhost:4443 -p gcs --gcs-content-type="application/json" ./test.json
5. Test filebeat with following filebeat.yml

filebeat.inputs:
- type: gcs
  project_id: testproject
  parse_json: true
  alternative_host: "http://localhost:4443/"
  buckets:
  - name: testbucket

The expected result is that a the object is retrieved and that parse_json works as expected

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[ShourieG]

LGTM

[elasticmachine]

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• Start Time: 2023-01-30T10:11:42.402+0000

• Duration: 71 min 45 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2547
Skipped	172
Total	2719

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[elasticmachine]

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• Start Time: 2023-01-30T10:10:40.545+0000

• Duration: 72 min 20 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2547
Skipped	172
Total	2719

🤖 GitHub comments

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To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)