Allow XmlDocumentReader to skip elements not in schema instead of fail

[mherwege]

See discussion openhab/openhab-addons#16060 (comment)

To my knowlege, all reading of xml files will fail if the xml file contains elements that are not in the schema (and code).

For 4.1, we enhance the addon.xml schema, adding discovery-methods. This works fine in 4.1. But it has been brought to our attention that it does mean bindings compiled for 4.1 will not work anymore on a 4.0 installation, even when all else would, this due to the fact that discovery-methods is not recognized.

It is too late to solve this for 4.0, but to avoid such situations in the future (when something else would be added), it would be nice of we could log and ignore the extra elements, instead of failing completely.

This requires changing the core to not throw an error on unrecognized elements, but log and skip. Something similar could be required for the attribute checking.

I am OK trying a change on this, but want to validate that this is acceptable before doing so. The current check is very strict and this may have broad consequences.

[andrewfg]

@mherwege I would support this.