Add extension number for protoc-gen-validate

This project is used by the [Envoy proxy](https://www.envoyproxy.io/) and several others.

The existing code is using the extension value [919191](https://github.com/envoyproxy/protoc-gen-validate/blob/54b08a686ff6548d8a248218664254c422fc4599/validate/validate.proto#L29) but since maintaining binary compatibility is not important, it shouldn't be a problem to change once this allocation is approved.

docs/options.md

@@ -197,3 +197,7 @@ with info about your project (name and website) so we can add an entry for you.
    * Website: https://github.com/container-storage-interface/spec
    * Extensions: 1059-1069
 
+1. Protoc-gen-validate
+   * Website: https://github.com/envoyproxy/protoc-gen-validate
+   * Extensions: 1070
+