Spec SAS verification and the common key verification framework

Reference implementations:
* 94f664e725
* https://github.com/matrix-org/matrix-react-sdk/pull/2461
* https://github.com/matrix-org/matrix-js-sdk/pull/818
* https://github.com/matrix-org/matrix-react-sdk/pull/2596
* https://github.com/matrix-org/matrix-js-sdk/pull/837

Proposals:
* [MSC1717](https://github.com/matrix-org/matrix-doc/pull/1717)
* [MSC1267](https://github.com/matrix-org/matrix-doc/issues/1267)

No alterations to either proposal have been made intentionally here.

event-schemas/schema/m.key.verification.start

@@ -0,0 +1,39 @@
+---
+allOf:
+  - $ref: core-event-schema/event.yaml
+
+description: |-
+  Begins a key verification process. Typically sent as a `to-device`_ event.
+properties:
+  content:
+    properties:
+      from_device:
+        type: string
+        description: |-
+          The device ID which is initiating the process.
+      transaction_id:
+        type: string
+        description: |-
+          An opaque identifier for the verification process. Must be unique
+          with respect to the devices involved. Must be the same as the
+          ``transaction_id`` given in the ``m.key.verification.request``
+          if this process is originating from a request.
+      method:
+        type: string
+        description: |-
+          The verification method to use.
+      next_method:
+        type: string
+        description: |-
+          Optional method to use to verify the other user's key with. Applicable
+          when the ``method`` chosen only verifies one user's key.
+    required:
+      - from_device
+      - transaction_id
+      - method
+    type: object
+  type:
+    enum:
+      - m.key.verification.start
+    type: string
+type: object