SuiteSparse:GraphBLAS includes many extensions to the official GraphBLAS specification, and one can reasonably expect other implementations (a.k.a. backends) to offer their own (possibly different) additions. We should find a non-intrusive yet clear way to support these.

When creating new objects, we can accept **backend_kwargs where valid keywords are determined and handled by the specific backend. For example, when creating a new matrix, we may have ss_format='by_row'. In this example, we have all SuiteSparse extension keywords begin with ss_.

Similarly, backends may want to have additional methods or properties. For SuiteSparse, this may look like A.ss.select(...), A.ss.format, A.ss.pattern, etc.