[8.15](backport #6028) Add var generation benchmark

[mergify]

What does this PR do?

Add a benchmark for variable generation from provider mappings. We specifically profile having a lot of Pods from the Kubernetes provider, which is something that has recently become a problem, and which the configuration generation pipeline doesn't deal with very well. I used test data from a real environment where the problems outlined in #5991 and #5835 could be reproduced.

Results on main:

goos: linux
goarch: amd64
pkg: github.com/elastic/elastic-agent/internal/pkg/composable
cpu: 13th Gen Intel(R) Core(TM) i7-13700H
BenchmarkGenerateVars100Pods-20    	      74	  15310045 ns/op	10527199 B/op	  201283 allocs/op
BenchmarkGenerateVars100Pods-20    	      74	  15185648 ns/op	10514404 B/op	  201271 allocs/op
BenchmarkGenerateVars100Pods-20    	      86	  15126049 ns/op	10521262 B/op	  201277 allocs/op
BenchmarkGenerateVars100Pods-20    	      87	  15067870 ns/op	10514019 B/op	  201275 allocs/op
BenchmarkGenerateVars100Pods-20    	      85	  15032633 ns/op	10515568 B/op	  201274 allocs/op
BenchmarkGenerateVars100Pods-20    	      90	  15185170 ns/op	10525087 B/op	  201282 allocs/op
BenchmarkGenerateVars100Pods-20    	      75	  15345854 ns/op	10516840 B/op	  201274 allocs/op
BenchmarkGenerateVars100Pods-20    	      80	  15908398 ns/op	10518576 B/op	  201277 allocs/op
BenchmarkGenerateVars100Pods-20    	      73	  15623320 ns/op	10529545 B/op	  201284 allocs/op
BenchmarkGenerateVars100Pods-20    	      86	  15946078 ns/op	10533571 B/op	  201286 allocs/op
PASS
ok  	github.com/elastic/elastic-agent/internal/pkg/composable	12.832s

Why is it important?

I'd like to make some improvements to both the var generation and the configuration generation in the coordinator, and want a straightforward way of demonstrating their impact.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas

Related issues

• Relates Elastic agent uses too much memory per Pod in k8s #5835
• Relates Memory Consumption Issue with Elastic Agent on Kubernetes with high number of resources #5991

---

This is an automatic backport of pull request #6028 done by [Mergify](https://mergify.com).

[elasticmachine]

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

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube