TUN-9007: modify logic to resolve region when the tunnel token has an endpoint field

## Summary

Within the work of FEDRamp it is necessary to change the HA SD lookup to use as srv `fed-v2-origintunneld`

This work assumes that the tunnel token has an optional endpoint field which will be used to modify the behaviour of the HA SD lookup.

Finally, the presence of the endpoint will override region to _fed_ and fail if any value is passed for the flag region.

Closes TUN-9007

connection/connection.go

@@ -60,6 +60,7 @@ type Credentials struct {
 	AccountTag   string
 	TunnelSecret []byte
 	TunnelID     uuid.UUID
+	Endpoint     string
 }
 
 func (c *Credentials) Auth() pogs.TunnelAuth {
@@ -74,13 +75,16 @@ type TunnelToken struct {
 	AccountTag   string    `json:"a"`
 	TunnelSecret []byte    `json:"s"`
 	TunnelID     uuid.UUID `json:"t"`
+	Endpoint     string    `json:"e,omitempty"`
 }
 
 func (t TunnelToken) Credentials() Credentials {
+	// nolint: gosimple
 	return Credentials{
 		AccountTag:   t.AccountTag,
 		TunnelSecret: t.TunnelSecret,
 		TunnelID:     t.TunnelID,
+		Endpoint:     t.Endpoint,
 	}
 }