Improve support versioning Page Model API

The Page Model API (PMA) will have at some point in time incompatible changes, for example because we change URLs from absolute to fully qualified. Such changes are BC incompatible and require to be access via a new API version.

We have the option to support versioned API via the PathInfo, a QueryString or a Header. For the PMA, most likely a header is easiest.

Note that the PMA version is NOT the same as the BRX version: typically, a single BRX version, eg 14.1.0, must be able to serve multiple PMA versions (say 1.0 and 2.0) : This way, BRX can be upgraded to a new version, while a customer her SPA still connects to the old PMA