DDS: Validate ROS_DOMAIN_ID environment is the same as DDS_DOMAIN_ID

## Feature request

**Is your feature request related to a problem? Please describe.**

If someone sets the ROS_DOMAIN_ID and it doesn't match their DDS_DOMAIN_ID, they can't communicate with the vehicle over DDS

**Describe the solution you'd like**

In `sitl.launch.py` , use [pymavlink to read the DDS_DOMAIN_ID param](https://www.ardusub.com/developers/pymavlink.html#read-and-write-parameters), and issue a warning if it doesn't match the environment variable ROS_DOMAIN_ID

**Describe alternatives you've considered**

The wiki explains this, but people miss it from time to time

**Platform**
[ x ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

**Additional context**

Start by reading sitl.launch.py and the linked pymavlink parameter-reading documentation to understand the existing launch flow and DDS_DOMAIN_ID lookup. Verify how ROS_DOMAIN_ID is obtained from the environment. Done means SITL warns when the two values differ, without changing behavior when they match.

Written by the indexing model from the issue text.