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.

[ramanathan1504]

@kamilkrzywanski Thanks for the Pr. Give some time i will review this ASAP

[ramanathan1504]

@kamilkrzywanski

Tests look great and passed locally.
I noticed the Spotless formatting check failed, so I just pushed a quick commit to your branch to apply the formatting. For future PRs, just make sure to check out the BUILDING.adoc before committing so you can run the Spotless checks locally.

LGTM 👍🏾