Accept the use of async predict methods

[RobertLucian]

This fixes the issue when a request has the content type set to multipart/form-data and a file is uploaded (i.e. when making a request of this kind curl <api-endpoint> -X POST -H "Content-Type: multipart/form-data" -F "iris=@sample.json").

This is because reading a form file requires the call of an async method read, which cannot be called unless the predict method of the predictor is an async method too, which in turn, means, the backend method must be an async function as well.

Here's an example of how to read a file using forms (scroll to the bottom of the page): https://www.starlette.io/requests/.

Edit

This is no longer required because the file can just be retrieved from the payload.file attribute, which is already a tempfile.SpooledTemporaryFile object. As shown in #1566. That being said, this PR might still be of interest to those that use async functions.

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checklist:

• run make test and make lint
• test manually (i.e. build/push all images, restart operator, and re-deploy APIs)