Add Olm unwedging

As per [MSC1719](https://github.com/matrix-org/matrix-doc/pull/1719)

No known alterations have been made to the proposal.

Implementation proof: https://github.com/matrix-org/matrix-js-sdk/pull/780

event-schemas/examples/m.dummy

@@ -0,0 +1,4 @@
+{
+  "content": {},
+  "type": "m.dummy"
+}

event-schemas/schema/m.dummy

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+---
+allOf:
+  - $ref: core-event-schema/event.yaml
+
+description: |-
+  This event type is used to indicate new Olm sessions for end-to-end encryption.
+  Typically it is encrypted as an ``m.room.encrypted`` event, then sent as a `to-device`_
+  event.
+
+  The event does not have any content associated with it. The sending client is expected
+  to send a key share request shortly after this message, causing the receiving client to
+  process this ``m.dummy`` event as the most recent event and using the keyshare request
+  to set up the session. The keyshare request and ``m.dummy`` combination should result
+  in the original sending client receiving keys over the newly establish session.
+properties:
+  content:
+    properties: {}
+    type: object
+  type:
+    enum:
+      - m.dummy
+    type: string
+type: object