[receiver/windowseventlogreceiver] Use a map for event_data

[strawgate]

Description

Changes the event_data format from an array of single-key maps (which is extremely difficult to work with in OTTL) to a flat map , making fields directly accessible via OTTL.

Before:

{
  "event_data": {
    "data": [
      {"ProcessId": "7924"},
      {"Application": "app.exe"},
      {"": "anonymous_value"}
    ]
  }
}

After:

{
  "event_data": {
    "ProcessId": "7924",
    "Application": "app.exe",
    "Data1": "anonymous_value"
  }
}

• Named <Data> elements become direct keys
• Anonymous <Data> elements get numbered keys (Data1, Data2, etc.)
• Fields now directly accessible: body["event_data"]["ProcessId"]

For Windows logs, any given log has either named elements or anonymous elements, which avoids typical key conflicts.

Link to tracking issue

Fixes #42565 and #32952

Testing

Unit tests updated. To test manually on Windows:

1. Build collector with changes
2. Configure receiver with raw: false (default)
3. Generate Windows events and verify event_data fields are flat keys

Note: Only applies when raw=false. When raw=true, body is XML string.

Documentation

Updated readme and added changelog