Provide possibility to merge phasedData properties

Whenever we perform export phased data the phasedData file we create is replaced with the new information.

One suggestion is to have a merge option. That if the file already exists we just enrich it. I.e.
* For the same property , we replace it
* For a new property we add it

This should be managed with a global variable sysem property, so when set to true we keep the existing properties

Relevance: when executing phased tests in parallel suite threads

Issue proposed by @priyankasaini077

Start by locating the phased data export entry point and the existing global system-property handling. Trace how an existing phasedData file is replaced, then verify that the configured merge behavior replaces matching properties while retaining existing ones and adds new properties, including when phased tests run in parallel.

Written by the indexing model from the issue text.