Include free-form message when encoding StructuredDataMessage to XML

[kamilkrzywanski]

What

Fixes #4141: StructuredDataMessage XML encoding omitted the free-form message supplied at construction.

Changes

• Encode the constructor message field as a distinct <message> element in asXml, with the same XML escaping used for type and id
• Keep it separate from map entries so a user-supplied put("message", ...) remains an independent SD-PARAM
• Omit the element when the constructor message is null
• Add regression tests for inclusion, escaping, and the map-key conflict case

Example

new StructuredDataMessage("an id", "a message", "a type");

now encodes as:

<StructuredData>
<type>a type</type>
<id>an id</id>
<message>a message</message>
<Map>
</Map>
</StructuredData>

When both the constructor message and a map entry keyed "message" are set, both appear:

<message>a message</message>
...
  <Entry key="message">foo</Entry>

Testing

mvn -pl log4j-api-test -am test -Dtest=StructuredDataMessageTest -Dsurefire.failIfNoSpecifiedTests=false

All StructuredDataMessageTest cases pass.