Fix deserialization failure in Change Metadata

[omar-ahmed42]

Description

Problem

• When a user attempted to add custom metadata (in JSON format), the user receives 400 with an exception indicating failure to convert java.lang.String to java.util.Map
• When a user attempted to add custom metadata (in frontend), after submission, no change occurred in the metadata as the Map was empty.

Solution

• Implement a custom property editor to convert string (allRequestParams) to Map, this way the user wouldn't get 400 anymore.
• Bind input field names with the Map by using the format for setting Map names (mapName[key] ), example:
  allRequestParams[customKey1], allRequestParams[customValue1].

Closes #2346

Checklist

• I have read the Contribution Guidelines
• I have performed a self-review of my own code
• I have attached images of the change if it is UI based
• I have commented my code, particularly in hard-to-understand areas
• If my code has heavily changed functionality I have updated relevant docs on Stirling-PDFs doc repo
• My changes generate no new warnings
• I have read the section Add New Translation Tags (for new translation tags only)