Add optional subscription channel size

[michel-laterman]

What is the problem this PR solves?

Daily 10k scale tests seems to still be flakey on the multiple policy change test.
This is meant as another simple solution before we try to add early cancelations to dispatching policy changes.

How does this PR solve the problem?

Add an optional arg when subscribing to a subscription monitor to specify the channel size. Min val of 1 is enforced. Use in policy monitor to allow more buffered changes to be queued as mutex might be help and block other dispatches.

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 Policy monitor load issues #3254

[michel-laterman]

buildkite run perf-tests

[elastic-sonarqube]

Quality Gate passed

The SonarQube Quality Gate passed, but some issues were introduced.

1 New issue
0 Security Hotspots
88.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[michel-laterman]

the 10k serverless tests succeeded, but one of the pods went into a crashloop; this is not a good approach to resolve the issue.