text: Title: Make the custom `Base64` type compatible with OpenAPI generation
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  ## Bug description
  
  <!-- A clear and concise description of what the bug is. -->
  
  It is currently not possible to define a FastAPI endpoint that includes our custom `Base64` type, https://github.com/RobertCraigie/prisma-client-py/issues/318#issuecomment-1214218519.
  
  ## How to reproduce
  
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  See https://github.com/RobertCraigie/prisma-client-py/issues/318#issuecomment-1214393759
  
  ## Expected behaviour
  
  <!-- A clear and concise description of what you expected to happen. -->
  
  FastAPI should generate an OpenAPI spec converting `Base64` inputs to a `string`.
  