This commit introduces the ability to handle custom requests in the
protocol package. A new struct, CustomRequestHandler, has been added
which contains a Method and a Func. The Func is a function that takes
a context and params and returns an error.

The Handler struct now includes a slice of CustomRequestHandlers, and
the Handle method has been updated to handle custom requests by
unmarshalling the params into the handler's params and then calling
the handler's Func.

A new method, FindCustomRequestHandler, has been added to the Handler
struct. This method takes a method string and returns the corresponding
CustomRequestHandler and a boolean indicating whether the handler was
found.

The server's handle method has also been updated to handle errors from
the Handler's Handle method.

The return type of CustomRequestFunc in both protocol_3_16 and
protocol_3_17 has been changed from error to (any, error). This
allows the function to return a result alongside an error. The
Handler's Handle method has been updated to accommodate this
change.

The 'params' field in the CustomRequestHandler struct has been renamed
to 'Params' and made public. This change is reflected in the associated
methods where 'params' was used. The 'protocol_3_17/custom.go' file has
been deleted. The 'FindCustomRequestHandler' method
now returns a 'protocol316.CustomRequestHandler' instead of a
'CustomRequestHandler'.