Implement OTEL secrets replacement in parsed policy

[michel-laterman]

What is the problem this PR solves?

Added functionality to replace secrets in OTEL sections (receivers, exporters, processors, extensions, connectors) of a policy. Updated the NewParsedPolicy function to process these sections and append identified secret keys. Included a new test case to validate the correct replacement of secrets in the OTEL sections.

How does this PR solve the problem?

Add a call to the existing ProcessMapSecrets method for OTEL keys in the policy.

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

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 Secrets in Fleet-managed OTel configuration are not rendered #6277

[github-actions]

@Mergifyio backport 9.2 9.3

[mergify]

backport 9.2 9.3

✅ Backports have been created

Details

• #6469 [9.2](backport #6419) Implement OTEL secrets replacement in parsed policy has been created for branch 9.2 but encountered conflicts

Cherry-pick of 9df10d4 has failed:

On branch mergify/bp/9.2/pr-6419
Your branch is up to date with 'origin/9.2'.

You are currently cherry-picking commit 9df10d4.
  (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:
	new file:   changelog/fragments/1771972977-Add-support-for-OTEL-secrets-handling.yaml
	new file:   internal/pkg/policy/testdata/policy_with_otel_secrets.json
	new file:   internal/pkg/server/otel_policy_secrets_integration_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   internal/pkg/policy/parsed_policy.go
	both modified:   internal/pkg/policy/parsed_policy_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

• #6470 [9.3](backport #6419) Implement OTEL secrets replacement in parsed policy has been created for branch 9.3