FEAT: Replacing NormalizerRequestPieces with SeedPrompts

[rlundeen2]

A good way to visualize how this is useful is to imagine sending a bunch of multi-modal prompts.

Before this change, you would load SeedPrompts from somewhere then have code to convert them to NormalizerRequestPieces, and then send them.

Now, you can load SeedPrompts from somewhere and send them without converting them first.

In summary, this PR does the following:

• Removing NormalizerRequestPiece so the normalizer can send SeedPrompts without the extra step
• Refactoring interfaces so they use SeedPrompts and they are easier to load
• Updates how the normalizer handles converters. Namely, request and response converters are now consistent and can be applied in different ways (e.g. by index, data_type, etc). Previously, request converters were configured using the now deleted NormalizerRequestPiece
• Tests updated