feat: add CPU profiling pull mode

[arnabnandy7]

Summary

Adds an opt-in pull mode that lets collectors request CPU profiles over HTTP in pprof format.

The implementation builds on the existing configuration and profiler interfaces, with bounded recording durations, concurrent-request protection and lifecycle cleanup. Existing push behavior remains the default.

This initial version supports async-profiler CPU profiling on Linux and macOS. Dynamic labels and trace context are not included.

Includes setup documentation, an Alloy example and focused tests. Compilation and packaging passed; tests and end-to-end collector verification remain pending.

Closes #13

Test Summary:

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[grcevski]

I think there's a lot of code here that needs to be better tested. For example the pprof encoding. I think we need some sort of proof that those encoders work properly for pprof.

Also, is it possible for you to split the pull request commits by concern and commit piece by piece with the tests.

[arnabnandy7]

I think there's a lot of code here that needs to be better tested. For example the pprof encoding. I think we need some sort of proof that those encoders work properly for pprof.

Also, is it possible for you to split the pull request commits by concern and commit piece by piece with the tests.

It'll take sometime but I'll split it here @grcevski thanks for your reviews.

[arnabnandy7]

@grcevski it's done, please review