[REQ] Improve endpoint type hint, body type should depend upon content_type input

[spacether]

Is your feature request related to a problem? Please describe.

Improve endpoint type hint, body type should depend upon content_type input

Describe the solution you'd like

• if there is only one content type, then the type hint for body should be correct without it
• when content_type is passed in, the type hint for body should be correct

Describe alternatives you've considered

Additional context