webhook: added delete all command
webhook: added delete all command

- allows mass deletion of all webhook
- logic might need to be improved. current might not be async?
- test to be implemented for completeness
- docs PR will be required as well

Signed-off-by: Kenneth Lim <9e44d7eb6fcab62e2399c89db54e6ef42f0711a1@gmail.com>

spark/particle-cli,kennethlimcp/particle-cli,kennethlimcp/particle-cli,spark/particle-cli,spark/particle-cli,kennethlimcp/particle-cli,spark/particle-cli,kennethlimcp/particle-cli,spark/particle-cli,kennethlimcp/particle-cli,kennethlimcp/particle-cli,spark/particle-cli