Potential improvements to identifying customers by external_id (WIP)

[reichert621]

Description

WIP

(TODO: write down thoughts on how we could improve this further)

Some things to keep in mind:

• We'll need to make sure this works in both the chat widget and the Storytime library
  • (Starting to think that it might be worth considering bundling these together somehow)
• How do we avoid duplicate customers with the same email/external_id? Is this a problem or not? (Seems like it would be a problem, but maybe it's ok across different domains?)
• Should we change how we cache customer info?

Issue

#515

Checklist

• Everything passes when running mix test
• Ran mix format
• No frontend compilation warnings