Added SYSTEM STOP/START PULLING REPLICATION LOG Queries for ReplicatedMergeTree Testing

Why it matters

These commands allow users to temporarily pause and resume the pulling of replication logs in ReplicatedMergeTree tables. This feature is valuable for testing replication behavior, debugging replication issues, and simulating replication stoppage without impacting the entire system.

How to use it

Use the following queries to control replication log pulling:
- To stop pulling replication logs:

SYSTEM STOP PULLING REPLICATION LOG

- To resume pulling replication logs:

SYSTEM START PULLING REPLICATION LOG

These commands can be run on the server to test or simulate replication operations on ReplicatedMergeTree tables.