[PACE-55] Improve hash transform

[ivan-p92]

The current hash transform is implemented in the platform-agnostic DefaultProcessingPlatformTransformer. There are a couple issues with it:

• The hashing functions differ across platforms, e.g. in their return types.
• Hashing functions aren't really stable: the output type is typically a number or bytes, regardless of the input. This means that if a policy is created where for some principals the original value is returned, but for other the hashed value, a data type mismatch occurs.

The first issue can be resolved by creating platform-specific implementations, which is straightforward with the recent ProcessingPlatformTransformer interface.

The second issue requires additional thought. The hashed value could perhaps be cast to the original datatype or so. In the meantime, platform-specific hashing can be implemented using the SQL Statement transform.

_PACE-56