[8.16](backport #6114) Store AST directly in provider mappings

[mergify]

What does this PR do?

For all variable provider types, we store mappings as untyped dictionaries and only convert them to AST when we're about to generate actual vars. This means we have to recompute all of it every time there's a change to any of the provider data, even if all the other data is unaffected. For example, if a Kubernetes Pod gains a new label, we need to regenerate vars for all Pods.

Instead, we can store mappings as AST to begin with. Then the only cost of emitting new vars after a change is updating a bunch of references.

Why is it important?

This unnecessary recomputation 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_dynamic.txt          │
                       │     sec/op     │   sec/op     vs base                │
GenerateVars100Pods-20     755.49µ ± 3%   76.87µ ± 2%  -89.82% (p=0.000 n=10)

                       │ bench_main.txt │          bench_dynamic.txt           │
                       │      B/op      │     B/op      vs base                │
GenerateVars100Pods-20    735.00Ki ± 0%   98.69Ki ± 0%  -86.57% (p=0.000 n=10)

                       │ bench_main.txt │          bench_dynamic.txt          │
                       │   allocs/op    │  allocs/op   vs base                │
GenerateVars100Pods-20     21.335k ± 0%   1.364k ± 0%  -93.61% (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

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

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This is an automatic backport of pull request #6114 done by [Mergify](https://mergify.com).

[elasticmachine]

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

[elastic-sonarqube]

Quality Gate failed

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

See analysis details on SonarQube