Add Pydantic Model Validation for CloudEvents

As a developer, 
I would like to use Pydantic models for automatic validation of CloudEvents in ucam-faas 
to ensure data integrity and ease of handling event data structures.

Background

The ucam-faas currently handles CloudEvents but lacks automatic validation of the event data structure. By integrating Pydantic models and adding decorators, we can validate the data payloads against predefined schemas, thus reducing errors and improving data handling. This enhancement will use the existing CloudEvent and CamSISCloudEventData schemas, generated Pydantic models, and extend functionality to support these validations.

Acceptance Criteria

• Implement a decorator @cloud_event_pydantic that wraps cloud event functions.
• The decorator should accept a Pydantic model class for validation.
• Raise appropriate validation errors if the data does not conform to the schema.
• Update README.md to include examples of using the new decorator.

Code Example

@ucam_faas.cloud_event_pydantic(CamSISCloudEventData)
def handle_camsis_event(camsis_event: CamSISCloudEventData):
   # CloudEvent and CamSISRecordExists validated and loaded in pydantic object
   do_something = camsis_event.xml_payload

https://gitlab.developers.cam.ac.uk/uis/devops/iam/idhub/api/-/merge_requests/60#note_637165