Validation: Allow to emit ValidationMessage for column/line of start element in DocumentViewXmlValidator.validateEnd

Currently all ValidationMessages emitted from https://github.com/apache/jackrabbit-filevault/blob/3774fdfef099aa3d8c55681c2590c652c99e97eb/vault-validation/src/main/java/org/apache/jackrabbit/vault/validation/spi/DocumentViewXmlValidator.java#L121 are automatically get the line/column of the ending tag in the XML file.
Sometimes one needs to emit messages which should rather refer to the position of the start tag (but in a deferred fashion, i.e. once all children have been visited).

This is useful to conditionally emit validations depending on child elements/nodes.
As the column/line is currently not passed via NodeContext nor any other argument of https://github.com/apache/jackrabbit-filevault/blob/3774fdfef099aa3d8c55681c2590c652c99e97eb/vault-validation/src/main/java/org/apache/jackrabbit/vault/validation/spi/DocumentViewXmlValidator.java#L89 it is currently not possible to explicitly set the message to a different line/column.