Add some target fields and move some things around (#3879)

In preparation for putting proxy code in the api package this creates some necessary fields in the target output and it pre-moves credential parsing and proxy handshake code to their new places. This will make the diff for the actual proxy code move quite a bit smaller.

Co-authored-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

api/targets/session_authorization.gen.go

@@ -21,5 +21,7 @@ type SessionAuthorization struct {
 	Type               string               `json:"type,omitempty"`
 	AuthorizationToken string               `json:"authorization_token,omitempty"`
 	Endpoint           string               `json:"endpoint,omitempty"`
+	EndpointPort       uint32               `json:"endpoint_port,omitempty"`
+	Expiration         time.Time            `json:"expiration,omitempty"`
 	Credentials        []*SessionCredential `json:"credentials,omitempty"`
 }