Remove race condition when accessing remote bulker map

[michel-laterman]

What is the problem this PR solves?

Remove a race condition/bug that may occur when remote ES outputs are used.

How does this PR solve the problem?

Change the bulkerMap and remoteOutputConfigMap attributes to sync.Map so concurrency controls are used for all interactions.
Change GetBulkerMap to return a copy of the map so that remote output health will not conflict with adding/removing a bulker from the map.

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 Test_Agent_Remote_ES_Output flaky due to race #4170

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[michel-laterman]

From what I can see, we had a flaky test because the policy self-monitor (internal/pkg/policy/self.go) was getting the bulker map in order to check remote output health; but we did not prevent our policy output preparation from creating a new bulker concurrently (internal/pkg/policy/policy_output.go).
I've changes our maps to sync.Map as we weren't properly using the mutex we had, and changed this func to return a copy instead

[elastic-sonarqube]

Quality Gate passed

Issues
1 New issue
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
55.6% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube