ddtrace/tracer: added UnmarshalJSON method to sampling rules

[dianashevchenko]

What does this PR do?

AIT-9799
This PR adds a UnmarshalJSON method on the SamplingRule struct. This is similar to unmarshalSamplingRules method which was used previously to process env variables.
Both methods utilise one validation function, however there's NO assertion on the rule type presence.

Motivation

Motivated by the dynamic sampling initiative, where one must be able to set sampling rules through an RC, thus requiring to unmarshal data from the request.

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.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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: 2024-02-19 12:34:52

Comparing candidate commit fc07b6d in PR branch shevchenko/sampling-unmarshal-method with baseline commit 655e11c in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 40 metrics, 1 unstable metrics.