[PROPOSAL] Move initialization into it's own function for AgoraClient

[Ascenio]

In the AgoraClient's constructor there is an implicit call to a initialization function.

https://github.com/AgoraIO-Community/Flutter-UIKit/blob/4ab388978f322caf99234420f5e8f510ebcd0a34/lib/src/agora_client.dart#L32-L44

If you dive more deep you see there are some async stuff going on which makes the constructor unexpectedly heavy.
Other packages have the same need (e.g.: camera and video_player) and do so by giving the client the work of calling initialize. It does seem more work from the client's perspective but it's just an one liner and gives the client the power to choose when to kick off the setup.