Fix checkin endpoint gzip handling

[michel-laterman]

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

[michel-laterman]

Some findings for validation of this:

• stdlib clients automatically add an Accept-Encoding: gzip header by default
• stdlib clients automatically handle responses that have a Content-Encoding: gzip
• the current behaviour of the server will server compress the checkin response if they are over 1kb uncompressed

Gathering data from the first couple of checkin an agent with the default policy (monitoring + systems integration) makes with a custom fleet-server build we can see the compressed resp savings, and estimate the request size savings to be:

checkin number	req body size	resp body size	req compressed	resp compressed	notes
1	1409	14783	780 (-44.6%)	3571 (-75.8%)	contains POLICY_CHANGE action
2	4588	35	1155 (-74.8%)	35 (no compression)	no returned actions

Note that these numbers are for agents running today, without effective_config as a part of the checkin request body.

[github-actions]

@Mergifyio backport 8.19 9.2 9.3

[mergify]

backport 8.19 9.2 9.3

✅ Backports have been created

Details

• #6526 [8.19](backport #6491) Fix checkin endpoint gzip handling has been created for branch 8.19 but encountered conflicts

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.
  (fix conflicts and run "git cherry-pick --continue")
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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

• #6527 [9.2](backport #6491) Fix checkin endpoint gzip handling has been created for branch 9.2
• #6528 [9.3](backport #6491) Fix checkin endpoint gzip handling has been created for branch 9.3