[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.
[PLAT-6805] Downcasting ON coupons to generate fixed-float swap definition

Basically mimicking the same behavior as for ibor coupons
[PLAT-6805] Adding checks to handles cases with stubs and notional exchanges properly