[8.16](backport #6184) Avoid unnecessary copies during config generation

[mergify]

What does this PR do?

Remove some unnecessary copies when generating component configuration.

The first copy avoided is done because we want to replace the inputs key with the rendered inputs. A shallow copy is sufficient for this. I did find and fix a bug in Dict shallow copies while at it.

The rest of the copies have to do with applying vars to each AST node. We currently make a copy of each Node we do this for, but that's not necessary, as Apply creates a new Node with all the data recursively copied anyway. I've added a test to verify that Apply actually works this way.

Benchstat result vs main, using the benchmark from #6180:

goos: linux
goarch: amd64
pkg: github.com/elastic/elastic-agent/internal/pkg/agent/application/coordinator
cpu: 13th Gen Intel(R) Core(TM) i7-13700H
                                      │ bench_main.txt │          bench_noclone.txt          │
                                      │     sec/op     │   sec/op     vs base                │
Coordinator_generateComponentModel-20     37.99m ± 24%   32.36m ± 4%  -14.82% (p=0.000 n=10)

                                      │ bench_main.txt │          bench_noclone.txt           │
                                      │      B/op      │     B/op      vs base                │
Coordinator_generateComponentModel-20     34.22Mi ± 0%   25.38Mi ± 0%  -25.84% (p=0.000 n=10)

                                      │ bench_main.txt │          bench_noclone.txt          │
                                      │   allocs/op    │  allocs/op   vs base                │
Coordinator_generateComponentModel-20      810.7k ± 0%   580.4k ± 0%  -28.40% (p=0.000 n=10)

Why is it important?

More performance is good! More broadly, this is an attempt to incrementally improve the problem from #5991.

Checklist

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

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

---

This is an automatic backport of pull request #6184 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 398f322 has failed:

On branch mergify/bp/8.16/pr-6184
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit 398f3229b8.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   changelog/fragments/1733158776-avoid-copies-config-gen.yaml
	modified:   internal/pkg/agent/application/coordinator/coordinator.go
	modified:   internal/pkg/agent/transpiler/ast_test.go
	modified:   internal/pkg/agent/transpiler/utils.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   internal/pkg/agent/transpiler/ast.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

[elastic-sonarqube]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 40%)

See analysis details on SonarQube