Cherry-pick #28695 to 8.0: [filebeat][httpjson] Set url as a pointer to ensure access to all methods

[marc-gr]

Cherry-pick of PR #28695 to 8.0 branch. Original message:

What does this PR do?

Change how url is stored in the request context, from value to pointer.

Why is it important?

In templates methods with a pointer receiver are not available if the variable is passed by value, making access to .url.Query and others impossible.

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.

[elasticmachine]

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

[elasticmachine]

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• Start Time: 2022-02-24T12:00:33.762+0000

• Duration: 102 min 15 sec

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Test	Results
Failed	0
Passed	2409
Skipped	159
Total	2568

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