Start annotating which version of the spec added a thing (#3425)

* Introduce a new "added-in" template and use it on endpoints

* Use "added-in" on schema properties too

* Annotate sections of the spec with their added versions

* Demo of "added-in" on a room version (to be fleshed out)

* Use clearer versioning semantics

* Update and fix validator for Swagger custom properties

* Fix docs

data/api/server-server/definitions/event-schemas/m.device_list_update.yaml

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+x-addedInMatrixVersion: "1.1"
 type: object
 title: m.device_list_update
 description: |-

data/api/server-server/definitions/event-schemas/m.signing_key_update.yaml

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+x-addedInMatrixVersion: "1.1"
 type: object
 title: m.signing_key_update
 description: |-