[TT-12814] Make schema more flexible, don't enforce additionalPropert…

…ies: false (#6640)

### **User description**
<details open>
<summary><a href="https://tyktech.atlassian.net/browse/TT-12814"
title="TT-12814" target="_blank">TT-12814</a></summary>
  <br />
  <table>
    <tr>
      <th>Summary</th>
<td>Make OAS JSON schema more flexible (do not enforce
additionalProperties)</td>
    </tr>
    <tr>
      <th>Type</th>
      <td>
<img alt="Bug"
src="https://tyktech.atlassian.net/rest/api/2/universal_avatar/view/type/issuetype/avatar/10303?size=medium"
/>
        Bug
      </td>
    </tr>
    <tr>
      <th>Status</th>
      <td>In Dev</td>
    </tr>
    <tr>
      <th>Points</th>
      <td>N/A</td>
    </tr>
    <tr>
      <th>Labels</th>
      <td>-</td>
    </tr>
  </table>
</details>
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https://tyktech.atlassian.net/browse/TT-12814


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### **PR Type**
Enhancement


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### **Description**
- Removed `additionalProperties: false` from multiple object definitions
in the JSON schema to enhance flexibility.
- This change allows additional properties in the schema, making it more
compatible and less strict.



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### **Changes walkthrough** 📝
<table><thead><tr><th></th><th align="left">Relevant
files</th></tr></thead><tbody><tr><td><strong>Enhancement</strong></td><td><table>
<tr>
  <td>
    <details>
<summary><strong>x-tyk-api-gateway.json</strong><dd><code>Relax schema
strictness by removing `additionalProperties:
false`</code></dd></summary>
<hr>

apidef/oas/schema/x-tyk-api-gateway.json

<li>Removed <code>additionalProperties: false</code> from multiple
object definitions.<br> <li> Enhanced flexibility of the JSON schema by
allowing additional <br>properties.<br>


</details>


  </td>
<td><a
href="https://github.com/TykTechnologies/tyk/pull/6640/files#diff-78828969c0c04cc1a776dfc93a8bad3c499a8c83e6169f83e96d090bed3e7dd0">+1/-73</a>&nbsp;
&nbsp; </td>

</tr>                    
</table></td></tr></tr></tbody></table>

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Co-authored-by: Tit Petric <tit@tyk.io>