[8.19](backport #6491) Fix checkin endpoint gzip handling

[mergify]

What is the problem this PR solves?

Checkin requests did not support "Content-Encoding: gzip" correctly.

How does this PR solve the problem?

Detect Content-Encoding: gzip header when validating checkin bodies and uncompress the body if detected.

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

  ---

  This is an automatic backport of pull request Fix checkin endpoint gzip handling #6491 done by Mergify.

[mergify]

Cherry-pick of 122fac1 has failed:

On branch mergify/bp/8.19/pr-6491
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit 122fac1.
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   changelog/fragments/1772577135-Fix-checkin-compression-support.yaml
	modified:   internal/pkg/api/handleCheckin.go
	modified:   internal/pkg/api/openapi.gen.go
	modified:   internal/pkg/server/fleet_integration_test.go
	modified:   model/openapi.yml
	modified:   pkg/api/client.gen.go
	modified:   pkg/api/types.gen.go

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