[8.x](backport #6585) Change the default gRPC port to 0 when in a container

[mergify]

What does this PR do?

Changes the default port used when agent runs in a container to port 0. There is at least one user whose rollout of Elastic Agent is experiencing collisions with another application on the same port (nobody else could ever pick 6789 as a default port surely...) and I thought this would be straight forward to fix quickly. I was wrong. I expected ~2 hours and spent 3 days:

1. It looked like we already have a separate default config file for Docker containers I could just update:

   elastic-agent/_meta/config/elastic-agent.docker.yml.tmpl

   Lines 88 to 89 in 9b8a25f

   	# # port for the GRPC server that spawned processes connect back to.
   	# port: 6789

2. This didn't work because the Helm chart defines its own configuration separately. I also didn't want to define this more than once because we'll probably change it again later (see note below):

   elastic-agent/deploy/helm/elastic-agent/templates/agent/k8s/_secret.tpl

   Line 5 in 9b8a25f

   agent.yml: |-

3. The run() function that is the entrypoint of the agent already has a hook for overriding configuration

   elastic-agent/internal/pkg/agent/cmd/run.go

   Lines 426 to 428 in 4199196

   	if override != nil {
   	override(cfg)
   	}

   that is already used in the container command to override the logging configuration

   elastic-agent/internal/pkg/agent/cmd/container.go

   Line 769 in 4199196

   func logToStderr(cfg *configuration.Configuration) {

   but of course updating this to change the gRPC port also didn't work.
4. It turns out we load the configuration from disk at least twice at startup, once in

   elastic-agent/internal/pkg/agent/cmd/run.go

   Line 415 in 9b8a25f

   cfg, err := configuration.NewFromConfig(rawConfig)

   and again in

   elastic-agent/internal/pkg/agent/application/application.go

   Line 99 in 4199196

   cfg, err := configuration.NewFromConfig(rawConfig)

   The second use didn't apply the overrides, the overrides only applied for the first case because that's when we created our logger and the only current overrides are for logging 🤦. Probably we could tidy this up but I've already spent enough time on this one.

note We should eventually stop using local TCP at all for the control protocol between sub-processes, and switch to unix sockets / named pipes. The capability for this was added in #4249, but it was left disabled because endpoint-security doesn't support it yet (because the upstream gRPC C++ client doesn't support it on Windows). We have now removed endpoint-security from our containers which would allow us to switch to unix sockets there, but this change required an elastic-agent-client package update and we need to test that every client we have has it first. This was more testing effort than I wanted to take on now, but I will create a follow up issue to do this.

Why is it important?

This automatically avoids port collisions between Elastic Agents using hostNetwork: true on Kubernetes as our DaemonSet does by default and other applications (or other Elastic Agents).

Disruptive User Impact

None, but in case I'm wrong about this I made it possible to choose a specific port with an environment variable.

How to test this PR locally

INSTANCE_PROVISIONER=kind SNAPSHOT=true GOTEST_FLAGS="-test.run TestKubernetesAgentHelm" TEST_PLATFORMS="kubernetes/arm64/1.31.0/basic" mage -v integration:kubernetes
```<hr>This is an automatic backport of pull request #6585 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elastic-sonarqube]

Quality Gate passed

Issues
3 New issues
1 Fixed issue
0 Accepted issues

Measures
0 Security Hotspots
52.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube