Adding new workflow for qml labs testing

[rashidnhm]

Context:
QML Labs Requires a separate CI workflow that runs separate from the current test suite of unit-tests on core pennylane.

This PR introduces the new workflow with the assumption of:

QML Labs tests will reside under:
pennylane/labs/tests/*

QML Labs source code will reside under:
pennylane/labs/*

Description of the Change:

• Coverage config is no longer a hard-coded env var in unit-tests. This allows us to have multiple coveragerc files.
• disable_new_opmath input has been removed and the jobs using it now pass it under pytest_additional_args instead.
• The job that stored tf/jax/torch tests are now in a separate workflow. This allows us to keep one source of truth for them and use them across core and labs tests.

Benefits:
Support for QML labs.

Possible Drawbacks:

Related GitHub Issues:
sc-76031

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.33%. Comparing base (cc80809) to head (718c480).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6441      +/-   ##
==========================================
- Coverage   99.70%   99.33%   -0.38%     
==========================================
  Files         448      447       -1     
  Lines       42566    42565       -1     
==========================================
- Hits        42441    42282     -159     
- Misses        125      283     +158     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.