Issue number: closes #8331

Summary

Changes

aws_lambda_powertools.utilities.data_classes.event_source complies with mypy in --strict mode as well.

Removed a redundant casting to the dataclass in examples/event_sources/src/kinesis_firehose_response_exception.py.

Changes are localised to event_source.py, rather than trying to annotate lambda_handler_decorator for all use cases, unlike earlier attempts, e.g. #4165. Since the decorator only takes the dataclass as a keyword argument, I've defined a protocol and cast the result to this.

User experience

Having a handler using the @event_source decorator, in main.py

from aws_lambda_powertools.utilities.data_classes import SQSEvent, event_source
from aws_lambda_powertools.utilities.typing import LambdaContext


@event_source(data_class=SQSEvent)
def lambda_handler(event: SQSEvent, _context: LambdaContext) -> None:
    return None

Before:

$ mypy --strict main.py

main.py:5:2: error: Untyped decorator makes function "lambda_handler" untyped  [untyped-decorator]

After:

$ mypy --strict main.py 

Success: no issues found in 1 source file

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