Added sd-channel-offset.json

[valkymaera]

added the extension json for sd-channel-offset
This adds the ability to modify individual noise channels before generation, via sliders.

Info

repo url: https://github.com/valkymaera/sd-channel-offset

Checklist:

• I have read the Readme.md
• The description is written in English.
• The index.json and extension_template.json have not been modified.
• The entry is placed in the extensions directory with the .json file extension.