Added `source_tag='canopy'` to `knowledge_base._get_global_client()`

[cwaddingham]

Problem

It's not easy to quickly attribute index activity to Canopy, so we don't have a reliable way of comparing its use with other frameworks using Pinecone.

Solution

This adds a source_tag, 'canopy', which is inline with how our partners manage their attributions.

Type of Change

• [ ] 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
• [ ] Infrastructure change (CI configs, etc)
• [ ] Non-code change (docs, etc)
• [ ] None of the above: (explain here)

Test Plan

1. Create a new Canopy index using the changed module.
2. Upsert one or more documents to the index.
3. Check the backend traces for the user_agent header "source_tag=canopy".

[cwaddingham]

@miararoy from our conversation last week.