Add Observe Active Cancel support to leshan-server-demo

[sbernard31]

This is about :

• add REST API to server demo.
• add UI to be able to call this API.

This will be useful for coap+tcp support (#1528) because passive cancel doesn't exist with coap+tcp : open-coap/java-coap#71

REST API is need for Zephyr integration tests : #1537.

I have not clear idea about how the UI could looks like, if you have idea do not hesitate to share.

[sbernard31]

REST API was added with commit : 2afadfc

API looks like :

# For "classic" observe : 
curl -i  -X DELETE  "http://{yourserver.url}/api/clients/{your_device_endpoint}/{your_node_path}/observe?active"
# Example : 
curl -i  -X DELETE  "https://leshan.eclipseprojects.io/api/clients/my_device/3442/observe?active"

# For "Composite" observe : 
curl -i  -X DELETE  "http://{yourserver.url}/api/clients/{your_device_endpoint}/composite/observe?paths={list_of_paths}&active"
# Example : 
curl -i  -X DELETE  "https://leshan.eclipseprojects.io/api/clients/my_device/composite/observe?paths=/3/0/1,/3/0/2,/3/0/13&active"

[SeppoTakalo]

@sbernard31 Thank you.
This seem to work. I can now do both, passive or active cancellations:

[sbernard31]

Great 👍
Thx a lot for taking time to validate that 🙏

[sbernard31]

UI using the new REST API was added by #1555.

[sbernard31]

#1555 is now integrated in master.