[9.1](backport #5858) Handle malformatted JSON in .fleet-agents components field

[mergify]

What is the problem this PR solves?

Handles a possible error in loading the model.Agent where the components field might be invalid JSON or a string (due to another bug). This changes the code to be more defensive, also should do less on parsing the components model when that is not needed.

How does this PR solve the problem?

Changes the components field to be a json.RawMessage that does two things. One it stops it from being parsed when the model.Agent is loaded from elasticsearch, which happens often and will reduce the time it takes to parse this field. Two, it stops that field from erroring to be unmarshalled when it is an invalid format.

How to test this PR locally

mage unitTest does a great job of covering it.

Checklist

• 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/fragments using the changelog tool

Related issues

• Closes findAgentByApiKeyId: could not unmarshal ES document into model.Agent: json: cannot unmarshal string into Go struct field Agent.components of type []model.ComponentsItem #5857

  ---

  This is an automatic backport of pull request Handle malformatted JSON in .fleet-agents components field #5858 done by Mergify.

[mergify]

Cherry-pick of f9dbe41 has failed:

On branch mergify/bp/9.1/pr-5858
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You are currently cherry-picking commit f9dbe41.
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Changes to be committed:
	new file:   changelog/fragments/1762447389-fix-issue-where-malformed-components-field-prevents-agent-authentication.yaml
	modified:   internal/pkg/api/handleCheckin_test.go
	modified:   internal/pkg/model/schema.go
	modified:   model/schema.json

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   internal/pkg/api/handleCheckin.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

[prodsecmachine]

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