[Tree] Make IdentifierMethod configurable when using xml config

[wouter-vs]

Feature Request

It is possible to define an identifier-method on tree-root when using annotations but not when using xml orm doctrine configuration. The functionality exists in the nested strategy but the xml config mapper doesn't read the option.

Adding this to the mapper would make it possible to use tree-root on a many-to-one field in xml config.

[github-actions[bot]]

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