MINOR: Move http method enum from proto_http to sample

MINOR: Move http method enum from proto_http to sample

This is useful to prevent cross includes. The header file sample.h
needs to include proto_http, stick_tables.h will need to include
sample.h and proto_http includes stick_tables.h.

I choose to move the known http method define because this enum is
mainly used in sample.h. This enum is used for the sample type method.