fix (ci): Allow opentelemetry-python forks to use CI

[jsoref]

trafficstars

Description

Adds CORE_REPO and CONTRIB_REPO inputs and corresponding environment variables to allow people to use CI in forks of https://github.com/open-telemetry/opentelemetry-python

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [x] Test A

Does This PR Require a Core Repo Change?

There is a corresponding change that should be made but it it can be done after this is merged.

• [ ] Yes. - Link to PR:
• [ ] No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• [ ] Followed the style guidelines of this project
• [ ] Changelogs have been updated
• [ ] Unit tests have been added
• [ ] Documentation has been updated