Add support for PFMERGE command

[Vsareen0]

Add support for the PFMERGE command in DiceDB similar to the PFMERGE command in Redis. Please refer to the following commit in Redis to understand the implementation specifics - source.

Write unit and integration tests for the command referring to the tests written in the Redis codebase 7.2.5. For integration tests, you can refer to the tests folder. Note: they have used TCL for the test suite, and we need to port that to our way of writing integration tests using the relevant helper methods. Please refer to our tests directory.

For the command, benchmark the code and measure the time taken and memory allocs using benchmem and try to keep them to the bare minimum.

[AshwinKul28]

Hey @Vsareen0 Do you have any updates on this? If you have any blockers lets discuss this over the discord. Or if you have multiple items on the plate, lets open this issue for everyone, and others can pick this up.
Thanks

cc - @JyotinderSingh

[lucifercr07]

Have started working on this as part of follow up PR to #422 (Adding PFCOUNT, PFADD)