Support solution in ommx format

[ksk-jij]

Description

At the moment, MathematicalProblemConverter does not support ommx.v1.Solution. Support it as it was supported.

Just Idea

MathematicalProblemConverter has decode function, however, to be fair, it is wrong (which is my decision since converters for QRAO is not the case for this decode, I'm sorry).

So,

1. Move the current decode to QAOAConverter and FQAOAConveter.
2. Define decode in MathematicalProblemConverter, which takes BinarySampleSet and returns ommx.v1.Solution.
3. Use MathematicalProblemConverter.decode in decode in QAOAConverter and FQAOAConverter, which means takes SampleResult[list[int]] and returns ommx.v1.Solution.