registration: Version the extension.json schema

registration: Version the extension.json schema

Versioning the extension.json schema will allow us to make breaking changes
to the schema in a non-breaking manner.

Extensiosn and skins will set a 'manifest_version' value, stating which
version of the schema the file is written for. Processor::extractInfo() will
be  passed the version number, and can switch behavior depending upon it.

For backwards-compatability, a version number of 1 is assumed if none is set.
The validateRegistrationFile.php script will emit a warning if this is the
case.

Bug: T99344
Change-Id: I2086a1465ceaeedd1ccc6804fda2c304ad16ffa0