Improve model alias support

[mitchstockdale]

Changes

• Adds support for validation_alias and serialization_alias for pydantic.Field (see Alias docs)
  • The behaviour added to SparkModel.model_spark_schema() is the same as that of pydantic.BaseModel.model_json_schema() (see docs)
  • 'validation' is the default mode by which to generate the schema ('serialization' is also supported)
  • by_alias is True by default
• Improves type annotations and variable names
• Clean up docstrings

Notes

• Tests could potentially be cleaned up and parameterised a bit better, but I opted for descriptive test function names instead. Happy to update these