feat: Add xml and lxml tree serializers

docs/data_binding/tree_serializing.md

@@ -0,0 +1,57 @@
+# Element Tree Serializer
+
+The element tree serializers will render an object into an element tree, that you can
+use to run XPATH evaluations or XSLT transformations.
+
+There are two implementations based on lxml
+[LxmlTreeSerializer][xsdata.formats.dataclass.serializers.LxmlTreeSerializer] and native
+python [XmlTreeSerializer][xsdata.formats.dataclass.serializers.XmlTreeSerializer].
+
+## xml.etree.ElementTree.Element
+
+```python
+>>> from xml.etree import ElementTree
+>>> from tests.fixtures.books.fixtures import books
+>>> from xsdata.formats.dataclass.serializers import XmlTreeSerializer
+...
+>>> serializer = XmlTreeSerializer()
+>>> result = serializer.render(books)
+...
+>>> result.find(".//title").text
+'The First Book'
+
+```
+
+## lxml.etree.Element
+
+```python
+>>> from lxml import etree
+>>> from tests.fixtures.books.fixtures import books
+>>> from xsdata.formats.dataclass.serializers import LxmlTreeSerializer
+...
+>>> serializer = LxmlTreeSerializer()
+>>> result = serializer.render(books)
+...
+>>> etree.indent(result)
+>>> actual = etree.tostring(result)
+>>> print(actual.decode())
+<ns0:books xmlns:ns0="urn:books">
+  <book>
+    <author>Hightower, Kim</author>
+    <title>The First Book</title>
+    <genre>Fiction</genre>
+    <price>44.95</price>
+    <pub_date>2000-10-01</pub_date>
+    <review>An amazing story of nothing.</review>
+  </book>
+  <book>
+    <author>Nagata, Suanne</author>
+    <title>Becoming Somebody</title>
+    <genre>Biography</genre>
+    <price>33.95</price>
+    <pub_date>2001-01-10</pub_date>
+    <review>A masterpiece of the fine art of gossiping.</review>
+  </book>
+</ns0:books>
+
+```

mkdocs.yml

@@ -116,6 +116,7 @@ nav:
   - JSON Parsing: data_binding/json_parsing.md
   - JSON Serializing: data_binding/json_serializing.md
   - Pycode Serializing: data_binding/pycode_serializing.md
+  - Tree Serializing: data_binding/tree_serializing.md
   - Dict Decoding: data_binding/dict_decoding.md
   - Dict Encoding: data_binding/dict_encoding.md
 - FAQ: faq.md