remoteconfig: add Subscribe function

[ahmed-mez]

What does this PR do?

Add a remote-config Subscribe function to register a product and its callback in a single call. This guarantees that the callback only receives updates for the corresponding product.

Motivation

Make the API more ergonomic and easier to use.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-11-24 12:01:27

Comparing candidate commit 37892ab in PR branch ahmed/rc-subscribe with baseline commit 8beb30d in branch main.

Found 3 performance improvements and 0 performance regressions! Performance is the same for 36 metrics, 2 unstable metrics.

scenario:BenchmarkSingleSpanRetention/no-rules-24

• 🟩 execution_time [-7.763µs; -6.715µs] or [-3.091%; -2.674%]

scenario:BenchmarkSingleSpanRetention/with-rules/match-all-24

• 🟩 execution_time [-8.078µs; -5.931µs] or [-3.190%; -2.342%]

scenario:BenchmarkSingleSpanRetention/with-rules/match-half-24

• 🟩 execution_time [-7.979µs; -6.071µs] or [-3.158%; -2.402%]