Feature Proposal: Add a Config Watcher/Listener for KV Change

[KunxiSun]

Currently, to react changes in configuration stored in Consul KV, applications using py-consul must implement manual polling logic.

I propose adding a ConfigWatcher function or a similar mechanism to the library. This feature would allow users to register a callback function to be invoked automatically whenever a specific key in the Consul KV store is modified.

Use Case Example-1:

import consul

c = consul.Consul()

@c.kv.watch('path/to/another/key')
def another_callback(old_value, new_value):
    print(f"Another key changed: {new_value}")

Use Case Example-2:

import consul

def my_smart_callback(old_value, new_value)
    print(f"Another key changed: {new_value}")

c = consul.Consul()
c.kv.watch('path/to/another/key', callback=my_smart_callback)

Key Features:

• Callback Registration:​ Register a Python callable (function/method) for a Consul KV key or prefix.
• Automatic Watch:​ The watcher should handle the long-polling logic, sessions, and index management internally, hidden from the user.

[KunxiSun]

@mbrulatout Hi! How do you think about this feature? Is there any better alternative solution?