[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.
It's right here as a private class. It's just a way for me to pass around these three elements that I need to properly build a Fixed Float SwapDefinition.

Basically there were two cases with almost identical code to generate that fixed float SwapDefinition. I'm adding a 3rd case but I was trying to avoid copy-pasting the code again. So I separated the code that pulls the fixed leg, the floating leg and the rate type. I placed these into my helper class. And from that I'm able to generate the proper SwapDefinition below.