feat: add strict unmarshalling options for LoadFunctionConfig and SimpleProcessor

- Introduced a new boolean field `Strict` in the SimpleProcessor struct to control strict unmarshalling behavior when loading function configurations.
- Updated the LoadFunctionConfig function to accept the `strict` parameter, allowing it to use strict unmarshalling.
- Modified SimpleProcessor to be able to control the behaviour
- Modified VersionedAPIProcessor, and TemplateProcessor to pass strict false.
- Added unit tests for LoadFunctionConfig to cover scenarios for strict mode, including successful loads and failure cases for unknown fields.

Author: lorenzofelletti