Add support for additional condition keys

[noamsdahan]

Adding support for additional condition keys, such as AWS global context keys or service-specific condition keys (KMS, IAM).

To add support for a condition key (that isn't multi-part, i.e. doesn't have / in it):

• Add a function to simulate_custom_policy_generator.py
def _get_aws_username(self) -> Optional[ContextEntryTypeDef]
  The function should return ContextEntryTypeDef or None if not applicable.
  If more information is needed from the event, add parsing that information to event.py.
• Add a reference to that function to KEY_FUNCTION_DICT
"aws:username": _get_aws_username,