[9.4](backport #6955) OpAMP redact slice maps

[mergify]

What is the problem this PR solves?

Redact slice maps with the form:

- name: Authorization
  value: secretValue

How does this PR solve the problem?

Use the shared redact package in elastic-agent-libs

How to test this PR locally

Enroll an opamp agent with sensitive values in the headers slices.

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

• Relates Move secret redaction logic to elastic-agent-libs elastic-agent-libs#415

  ---

  This is an automatic backport of pull request OpAMP redact slice maps #6955 done by Mergify.

[mergify]

Cherry-pick of 4d84c65 has failed:

On branch mergify/bp/9.4/pr-6955
Your branch is up to date with 'origin/9.4'.

You are currently cherry-picking commit 4d84c65.
  (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/1777917457-OpAMP-redact-sensitive-values-in-header-maps.yaml

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