Define genai content uploader hook and entry point

[aabmass]

Description

Introduces an interface for the generic GenAI upload hook. See SemConv uploading content to external storage. It also adds an entry point and environment variable to load an upload hook.

Instrumentations should load the hook with load_upload_hook() and call it before ending the span as described in semconv.

The upload hook implementor is responsible for implementing batching and backpressure if they choose to do so.

Part of #3065, #3753

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

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

• Added a unit test for the loading logic

Does This PR Require a Core Repo Change?

• 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 -- would prefer to update changelogs once the feature is done
• Unit tests have been added
• Documentation has been updated

[aabmass]

Thanks for the reviews. I tweaked this a little to expect the entrypoint to return a UploadHook class instance which has an upload() method.

[DylanRussell]

LGTM