rpcserver: return an encoded payment request in AddInvoice
rpcserver: return an encoded payment request in AddInvoice

This commit modifies the generated response to an “AddInvoice” RPC by
including an encoded payment request in the response. This change gives
callers a new atomic piece of information that they can present to the
payee, to allow completion of the payment in a seamless manner.

LightningNetwork/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,ddustin/lnd,LightningNetwork/lnd,bryanvu/lnd,bryanvu/lnd,aakselrod/lnd,AndrewSamokhvalov/lnd,ddustin/lnd