PLAT-5893 Set issuer id for FRNs to CUSIP stub to link to legal entities.  All other bond types have legalEntityId attribute set to string of ExternalId.

We're going to try linking Bloomberg loaded bills/frns/bonds with legal entity data using CUSIP stubs, the six character start of each CUSIP, which is unique to each issuer.
We'll have to see how well this works in practice.
Reviewed-By:Andrew Griffin
PLAT-5893 further tweaks from review by AG.