Make `ChargeBearer` configurable for SWIFT transfers

[cbm-egoubely]

Hello,

I noticed that the ChargeBearer for SWIFT transfers is currently hard-coded to SHAR (see: swift-credit-payment-initiation.ts#L263C13-L263C28).

It would be very helpful if this value could be made configurable, so that other valid options (DEBT, CRED, SLEV) can be used when needed.

Thanks a lot for the great work on this library! 🙏