fix: Add public Version for dd-trace-go for contrib

[e-n-0]

What does this PR do?

This PR adds a public method Version() to get the current version of the dd-trace-go package so this can be used in the contrib (envoy & request-mirroring contribs).

Motivation

The version of the tracer is incorrectly reported since v2 inside the contribs. This is making fails system-tests for the exteral-processing scenarios (that needs the go tracer version to correctly applies the tests manifest).

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.
• New code is free of linting errors. You can check this by running golangci-lint run locally.
• 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.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-06-06 14:12:38

Comparing candidate commit 312a63e in PR branch flavien/fix-contrib-version with baseline commit 2d915b3 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 21 metrics, 0 unstable metrics.