Move embeddings from issues index to workitems index

As Enable Issues Search and Epics from WorkItems I... (&14732 - closed) prepares to move data into the new workitems index in order to delete the issues and epics indices, we need to move embeddings over as well.

• Add mapping
• Add migration to move embeddings from one index to another or to backfill from scratch

Also change:

• dot-product to cosine
• maybe add task type: https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/text-embeddings-api