Add Call::refer

[kbalt]

No description provided.

[hkrutzer]

Apologies for the late reply @kbalt and thanks for this feature! I ran into some unrelated issues that I had to solve before I could test it. I would like to test it some more but the functionality works! I did have to make some local changes to make ReferError ReferEvent and EventSubscriptionReceiver public.

[hkrutzer]

Tested this out further and it is working well for me. This could be useful:

diff --git a/sip/sip-ua/src/lib.rs b/sip/sip-ua/src/lib.rs
index 916dfde8aa..9b494c81f6 100644
--- a/sip/sip-ua/src/lib.rs
+++ b/sip/sip-ua/src/lib.rs
@@ -28,7 +28,7 @@
 mod registration;
 mod subscription;

-pub use call::{Call, CallError, CallEvent};
+pub use call::{Call, CallError, CallEvent, ReferError};
 pub use inbound_call::{AcceptCallError, InboundCall, InboundCallFromInviteError, NoMedia};
 pub use media_backend::MediaBackend;
 #[cfg(feature = "rtc")]
@@ -36,4 +36,6 @@
     Codec, MediaEvent, RtcMediaBackend, RtcMediaBackendError, RtpReceiver, RtpSender,
 };
 pub use outbound_call::{MakeCallCompletionError, MakeCallError, OutboundCall};
+pub use refer::ReferEvent;
 pub use registration::{RegisterError, RegistrarConfig, Registration};
+pub use subscription::EventSubscriptionReceiver;

esp. the last two because I was unable to have the compiler infer let mut refer_receiver: Option<_> = None; to be let mut refer_receiver: Option<EventSubscriptionReceiver<ReferEvent>> = None;