[SDTEST-3172] Add runtime tags override to skip test files locally

[anmarchenko]

What does this PR do?
Adds a command line parameter and corresponding environment variable to provide an override for runtime tags collected by Datadog libraries.

It is important because when using Test Impact Analysis, skippable tests are scoped by runtime environment (OS, architecture, language version). This means tests skipped in the Linux CI won't automatically be skipped on the macOS development machine because the runtime tags differ.

The --runtime-tags option lets you override your local runtime tags to match your CI environment, enabling users to benefit from Test Impact Analysis locally without re-running their entire test suite.

The documentation about this feature is added to README.md

Motivation
A conversation with early adopters about a possibility to use ddtest to run a subset of tests in the local development environment

Additional Notes
Unfortunately, this will not make the Datadog Test Optimization library to skip the tests locally - but it can be used to skip locally the complete test files where all tests are skipped by test impact analysis

How to test the change?
Was manually tested using anmarchenko/forem project

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