Add Cumulocity as Leshan adopter.

Created by: sbernard31

I would like to add Cumulocity as Leshan adopters. (See https://github.com/eclipse/leshan/issues/830#issuecomment-792805687)

Cumulocity is done by Ag Software, discussing with us by email they prefer to put forward the "Cumulocity by AG Software" branding, I hope it's OK.

⚠ I was not able to test this PR locally ⚠, so please double check it 🙏

The reason : when I changed config\adopters.json file this doesn't changed the UI available at http://localhost:1313/adopters/#iot.leshan when I launch hugo server. I'm not sure If I found something wrong 🐛 or if It's just an issue with npm/node/hugo on my side which I'm not able to explain 🤔 (I tried to clear my browser cache just in case without any effect)

Bonus question, I was understanding that package-lock.json file is intended to be committed and it seems in this project this is not the case, is there any reason ?