[PLAT-6805] Adding support for Fixed/OIS swaps

Support for par rate calculation exists for all fixed-float swaps but
the converter from financial security to swap definition does not
process fixed-float swaps to a fixed-float swap definition if the index
is other than ibor.

This PLAT adds the support for swap with floating leg with OIS index.
I was trying to modify as few lines as possible. Here the logic is that if we don't fall into any of the special cases, we return a generic SwapDefinition.