Fix legend and feature info for wms difference output.

[na9da]

What this PR does

Makes following changes to WMS difference output:

• Fixes legend URL for WMS difference outputs - Fix legend URL for difference item generated using difference tool #7237
• Allow overriding properties of difference output item - Provide option to override traits of generated difference output #7238

Test me

1. Open this test link which has a difference output

2. Ensure the legend shown is for the difference output
   

3. Ensure the feature info for the difference item shows the raw data by default.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[nf-s]

Looks good!