Support both query parameters and body parameters for POST requests

[sallas]

With the newly added parse_parameters decorator, the logic assume that GET request has query parameters only and POST requests has body parameters only.

Based on comments from @sloev we believe it wouldn't be too difficult to add support for both when doing a POST request. More information about thoughts and potential solutions exists in the comments link below.

https://github.com/trustpilot/python-sanicargs/pull/14#discussion_r545719791 https://github.com/trustpilot/python-sanicargs/pull/14#discussion_r545725531