[Scheduling] Import/Export support for external problem formats

Now that the SSP dialect is starting to land, one potentially enhancement is to import/export CIRCT scheduling problems into and out of well-defined solver formats. For example, SAT problems have a CNF format, and SMT problems have SMT2 format, etc. This might not be directly useful for flows using CIRCT's scheduling library, but at least being able to export problems could open the door for such flows to kick out problems in a well-known format for solver researchers to experiment offline.

Start by reviewing the SSP dialect and the scheduling library context, then compare the SAT CNF and SMT SMT2 formats mentioned in the issue. Done means the supported external problem formats and corresponding import/export scope are clearly defined and implemented for CIRCT scheduling problems.

Written by the indexing model from the issue text.