Provide connection state API during reconnection attempts when disconnecting

[tartea]

Description:
During MQTT client startup, if the server is disconnected, the client automatically attempts to reconnect. If the application calls disconnect() during this reconnection phase, it only receives a "client is connecting" notification. This makes it difficult to properly handle disconnection during reconnection states.

Problem:
Currently, there's no way to:

Determine if the client is in a reconnection attempt state

Get the current connection state when disconnecting during reconnection

Properly synchronize disconnect operations with reconnection logic

Suggested Solution:
Provide an API to:

Expose the current connection state (including reconnection status)

Allow checking whether reconnection attempts are in progress