Introduce primitives for input/output dtype conversion in Lite Precision

[awaelchli]

What does this PR do?

The Precision plugin exposes API to convert model input and outputs to the appropriate precision dtype.
Note that this is just for Lite for now, as precision integration in PL is still ongoing.

Follow up to #14815

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @justusschock @awaelchli @rohitgr7 @carmocca @akihironitta

[carmocca]

Merged #14823

[carmocca]

Re-approving after the latest changes