Asset API

[cassiozen]

- Summary

Refactor on MediaProxy (now AssetProxy) to accept integrations that can upload the file, bypassing the original git file commit.

Assets will still be committed to git if no Asset Integration is provided.

Sample Config:

integrations:
  - provider: assetStore
    getSignedFormURL: https://smashing-proxy.herokuapp.com/assets
    hooks:
      - assetStore

- Test plan
WIP - Jest tests will be updated

- Description for the changelog
Optional integration for Asset uploads

- A picture of a cute animal (not mandatory but encouraged)

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