sip: Allow specifying if media encryption is desired

A property of type SipMediaEncryption is added to both the origin and
the call which allows to state if we want the media session to be
encrypted with SRTP.

Logic is added to interact with the CallsSdpCryptoContext if encryption
is desired.

plugins/sip/calls-sip-util.h

@@ -54,6 +54,18 @@ typedef enum {
   SIP_ENGINE_READY,
 } SipEngineState;
 
+/**
+ * SipMediaEncryption:
+ * @SIP_MEDIA_ENCRYPTION_NONE: Don't encrypt media streams
+ * @SIP_MEDIA_ENCRYPTION_PREFERRED: Prefer using encryption, but also allow unencrypted media
+ * @SIP_MEDIA_ENCRYPTION_FORCED: Force using encryption, drop unencrypted calls
+ */
+typedef enum {
+  SIP_MEDIA_ENCRYPTION_NONE = 0,
+  SIP_MEDIA_ENCRYPTION_PREFERRED,
+  SIP_MEDIA_ENCRYPTION_FORCED,
+} SipMediaEncryption;
+
 
 gboolean    check_sips                         (const char *addr);
 gboolean    check_ipv6                         (const char *host);