feat: allow `StatusCode()` for `error`s returned in Strict Server

[jamietanna]

To allow controlling what actually gets returned

[jamietanna]

May be trivial to do it yourself with the provided error handler

[deefdragon]

Returning an echo.ErrFoo as the error should properly be unwrapped and the correct response code returned.

Not sure if this is the ideal implementation tho as it leaks using echo into the strict implementation. Also not sure if that is worse that duplicating the already-existing error response codes.