Add channel version to content embed request schema

[bjester]

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Current behavior

The schema used to define requests which generate embeddings for content accept a channel_id.

Desired behavior

In addition to the channel_id, the request schema schema-embed_content_request.json should accept a integer channel_version argument.

Value add

Tracking the channel_id on the embeddings allows us to purge embeddings for old versions of the channel, like for content that was removed.