Move parse function to WPNetworkResponse

[oguzkocer]

• Moves parse_wp_response, parse_response_for_generic_errors functions and add_uniffi_exported_parser macro to WPNetworkResponse
• Adds a parse_with function to replace WPNetworkResponseParser trait
• Replaces String::from_utf8_lossy(&response.body).to_string() with body_as_string introduced in All public types implement Debug trait #109
• Moves RequestMethod enum to request module in 7b1bffa

I might do further improvements to this setup, but to keep the PR simple, this is mostly just moving things around.