Basic call transfer initiation support

Sends an m.call.replaces event and flag for whether to advertise
support.

MSC2747 (https://github.com/matrix-org/matrix-doc/pull/2747)

src/matrix.ts

@@ -142,6 +142,7 @@ export interface ICreateClientOpts {
     unstableClientRelationAggregation?: boolean;
     verificationMethods?: Array<any>;
     forceTURN?: boolean;
+    supportsTransfer?: boolean,
     fallbackICEServerAllowed?: boolean;
     cryptoCallbacks?: ICryptoCallbacks;
 }