add client lib

[LaurentMontBlanc]

The rddl-2-plmnt-service is and can be used for a multitude of token schemes. We need to add a client lib for easy maintenance of other services.

type IR2PClient interface {
  GetReceiveAddress(plmntAddress string) (ReceiveAddressResponse)
}

type R2PClient struct {
  host string
}

• [x] implement provided interface
• [x] define types
• [x] release package