New module: Support manipulating XML files (#25323)

* Import original unmodified upstream version

This is another attempt to get the xml module upstream.
https://github.com/cmprescott/ansible-xml/

This is the original file from upstream,
without commit 1e7a3f6b6e2bc01aa9cebfd80ac5cd4555032774

* Add additional changes required for upstreaming

This PR includes the following changes:

- Clean up of DOCUMENTATION
- Rename "ensure" parameter to "state" parameter (kept alias)
- Added EXAMPLES
- Remove explicit type-case using str() for formatting
- Clean up AnsibleModule parameter handling
- Retained Python 2.4 compatibility
- PEP8 compliancy
- Various fixes as suggested by abadger during first review

This fixes cmprescott/ansible-xml#108

* Added original integration tests

There is some room for improvement wrt. idempotency and check-mode
testing.

* Some tests depend on lxml v3.0alpha1 or higher

We are now expecting lxml v2.3.0 or higher.
We skips tests if lxml is too old.

Plus small fix.

* Relicense to GPLv3+ header

All past contributors have agreed to relicense this module to GPLv2+, and GPLv3 specifically.

See: https://github.com/cmprescott/ansible-xml/issues/113

This fixes cmprescott/ansible-xml#73

* Fix small typo in integration tests

* Python 3 support

This PR also includes:
- Python 3 support
- Documentation fixes
- Check-mode fixes and improvements
- Bugfix in check-mode support
- Always return xmlstring, even if there's no change
- Check for lxml 2.3.0 or newer

* Add return values

* Various fixes after review

test/integration/targets/xml/tasks/test-xmlstring.yml

@@ -0,0 +1,31 @@
+---
+  - name: Read from xmlstring
+    xml:
+      xmlstring: "{{ lookup('file', '{{ role_path }}/fixtures/ansible-xml-beers.xml') }}"
+      pretty_print: True
+    register: xmlresponse
+ 
+  - name: Write result to file
+    copy:
+      dest: /tmp/ansible-xml-beers.xml
+      content: "{{ xmlresponse.xmlstring }}"
+
+  - name: Test expected result
+    command: diff -u {{ role_path }}/results/test-pretty-print-only.xml /tmp/ansible-xml-beers.xml
+
+  - name: Read from xmlstring
+    xml:
+      xmlstring: "{{ lookup('file', '{{ role_path }}/fixtures/ansible-xml-beers.xml') }}"
+      xpath: /business/beers
+      pretty_print: True
+      add_children:
+        - beer: "Old Rasputin"
+    register: xmlresponse_modification
+ 
+  - name: Write result to file
+    copy:
+      dest: /tmp/ansible-xml-beers.xml
+      content: "{{ xmlresponse_modification.xmlstring }}"
+
+  - name: Test expected result
+    command: diff -u {{ role_path }}/results/test-pretty-print.xml /tmp/ansible-xml-beers.xml