feat: Improve the workflow of monitor / decrypting notifications

[Xlin123]

All of this work is up to discussion, I'm really not sure the best way to handle this stuff.

-What I did
I am mostly brainstorming on how to improve how we actually use monitor / decrypt notifications since right now it's a bit messy.

I created a AtNotificationService that would handle all of the monitor stuff. I left notify in at_client but we can easily move it later.

We now use a generator method in order to yield incoming at_events. In order to thoroughly implement notifications in an app I still suggest to use an additional thread to manage incoming notifications.

I modified handle_event a bit, there was 2 changes:

1. Added another Queue, decrypted_events, for specifically decrypted notifications.
2. Added the notify:remove command to get rid of notifications sitting around.

- How I did it

- How to verify it
I was testing it in parallel with sshnpdpy, so here are some examples on how to use the methods.

• Example on starting the monitor
• Example on decrypted notifications

I'll begin writing unit tests for this.

- Description for the changelog
feat: Improve the workflow of monitor / decrypting notifications

[Xlin123]

@realvarx I was thinking about abstracting _start_monitor, and _handle_event out to another class. (maybe a util class)?