[Automation] Update JSON server schema specs (#2542)

### What
APM agent json server schema automatic sync

### Why
*Changeset*
* elastic/apm-data#424
*
elastic/apm-data@184e8d7

---



<Actions>
<action
id="e779bb0c8f30a963db858c559b963edd1fa99844e8a5213fbc4f54b247007f2d">
        <h3>automation: update JSON server schema specs</h3>
<details
id="fa450b1827de4f90bb214eaf9924527a95b821d8f72de0b38a8f5b75ac6dcdc2">
<summary>APM agent json server schema
184e8d7fd0c3063688603db090f0c4c3850f93aa</summary>
<p>ran shell command &#34;tar -xzf
/home/runner/work/apm-agent-dotnet/apm-agent-dotnet/json-schema.tgz
&amp;&amp; git --no-pager diff&#34;</p>
        </details>
<a
href="https://github.com/elastic/apm-agent-dotnet/actions/runs/12782492313">GitHub
Action workflow link</a>
    </action>
</Actions>

---

<table>
  <tr>
    <td width="77">
<img src="https://www.updatecli.io/images/updatecli.png" alt="Updatecli
logo" width="50" height="50">
    </td>
    <td>
      <p>
Created automatically by <a
href="https://www.updatecli.io/">Updatecli</a>
      </p>
      <details><summary>Options:</summary>
        <br />
<p>Most of Updatecli configuration is done via <a
href="https://www.updatecli.io/docs/prologue/quick-start/">its
manifest(s)</a>.</p>
        <ul>
<li>If you close this pull request, Updatecli will automatically reopen
it, the next time it runs.</li>
<li>If you close this pull request and delete the base branch, Updatecli
will automatically recreate it, erasing all previous commits made.</li>
        </ul>
        <p>
Feel free to report any issues at <a
href="https://github.com/updatecli/updatecli/issues">github.com/updatecli/updatecli</a>.<br
/>
If you find this tool useful, do not hesitate to star <a
href="https://github.com/updatecli/updatecli/stargazers">our GitHub
repository</a> as a sign of appreciation, and/or to tell us directly on
our <a
href="https://matrix.to/#/#Updatecli_community:gitter.im">chat</a>!
        </p>
      </details>
    </td>
  </tr>
</table>

Co-authored-by: elastic-observability-automation[bot] <180520183+elastic-observability-automation[bot]@users.noreply.github.com>

src/Elastic.Apm.Specification/specs/span.json

@@ -188,6 +188,9 @@
                 "object"
               ],
               "properties": {
+                "body": {
+                  "description": "The http request body usually as a string, but may be a dictionary for multipart/form-data content"
+                },
                 "id": {
                   "description": "ID holds the unique identifier for the http request.",
                   "type": [