fix: create mutable copy of JSON serialisation object

[1abhishekpandey]

Description

• When someone calls the 'identify' event twice with the same userId and externalId (having the type field the same), then an error is thrown. The RSUtils.deserialise() method returns an immutable object for the externalId type, and when the code attempts to modify that immutable object, an error is thrown.
• Now we've made changes to allow the RSUtils.deserialise() to return the mutable object.
• Also, added a unit test case to simulate this scenario.

I've refactored the code to fix this merging issue.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud