Import the GCP resource detector class

[DylanRussell]

Description

Import a resource detector class for GCP. This change doesn't add any new code. It's re-exporting a package that is in the downstream GCP repo. This lets the GCP resource detector be loaded in via the "opentelemetry_resource_detector" entrypoint used here

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• 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)
• [X ] This change requires a documentation update

How Has This Been Tested?

n/a

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• [ X] No.

Checklist:

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

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

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[xrmx]

Why are we taking the burden of maintaining an empty package upstream just for a re-export? Why not defining the entry point in your package so that people interested can just use it?
I'm a user of the gcp resource detector and defining the entry point myself in my package.