Add NRGAccount to account claims

It is string format because it is quite likely that
we will extend the server in the future to be able to
move the traffic into a third named account.

For now expected values would be `"account"` or `"system"`.

Signed-off-by: Neil Twigg <[email protected]>

Make it a string instead

v2/account_claims.go

@@ -241,6 +241,7 @@ type Account struct {
 	Mappings           Mapping               `json:"mappings,omitempty"`
 	Authorization      ExternalAuthorization `json:"authorization,omitempty"`
 	Trace              *MsgTrace             `json:"trace,omitempty"`
+	NRGAccount         string                `json:"nrg_account,omitempty"`
 	Info
 	GenericFields
 }