[8.16](backport #6058) Don't deepcopy AST when generating vars

[mergify]

What does this PR do?

We generate vars from mappings from both context providers and dynamic providers. Dynamic providers can produce multiple var entries - the Kubernetes provider will do so for each Pod, for example. When that happens, we currently make a deep copy of the context provider mapping, and add the dynamic provider mapping as a key.

As we don't modify the mappings, this deep copy can be replaced by a shallow copy. This PR introduces shallow copying to the AST and uses it when generating vars, leading to a major performance improvement.

Why is it important?

All these deep copies can be quite expensive when there's a lot of mappings from the dynamic provider. See the benchstat report below:

goos: linux
goarch: amd64
pkg: github.com/elastic/elastic-agent/internal/pkg/composable
cpu: 13th Gen Intel(R) Core(TM) i7-13700H
                       │ bench_main.txt │          bench_shallow.txt          │
                       │     sec/op     │   sec/op     vs base                │
GenerateVars100Pods-20    15247.8µ ± 4%   778.8µ ± 7%  -94.89% (p=0.000 n=10)

                       │ bench_main.txt │          bench_shallow.txt           │
                       │      B/op      │     B/op      vs base                │
GenerateVars100Pods-20   10273.4Ki ± 0%   779.1Ki ± 0%  -92.42% (p=0.000 n=10)

                       │ bench_main.txt │          bench_shallow.txt          │
                       │   allocs/op    │  allocs/op   vs base                │
GenerateVars100Pods-20     201.28k ± 0%   22.13k ± 0%  -89.00% (p=0.000 n=10)

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

• 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 #6058 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of db6fbe2 has failed:

On branch mergify/bp/8.16/pr-6058
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit db6fbe2429.
  (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:
	modified:   internal/pkg/agent/transpiler/ast.go
	modified:   internal/pkg/agent/transpiler/ast_test.go
	modified:   internal/pkg/agent/transpiler/vars.go

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

[elasticmachine]

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

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/8.16/pr-6058 upstream/mergify/bp/8.16/pr-6058
git merge upstream/8.16
git push upstream mergify/bp/8.16/pr-6058

[elastic-sonarqube]

Quality Gate failed

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

See analysis details on SonarQube