pandora: implement L2CAP service

[uael]

No description provided.

[zxzxwu]

I really doubt whether Bumble should have such a fine-controlled interface. There is actually no demand to make such an almighty interface...

[uael]

The issue I'm trying to solve is that we want waiting for incoming connections to be reproducible, while right now it depends on good timing between the link connections and the channel connection request. From a DUT point of view, it's quite hard to make those assumptions and it's almost impossible to implement the Wait API for Fluoride, but from the REF perspective we need maximum control and reproducibility.

Other solutions could be to:

1. make the l2cap.WaitConnection scoped at the device level instead of connection level and add a l2cap.OwnedBy(channel, connection) API to match channels and connections.
2. add a l2cap.Register(spec) -> ServerToken API, then modify WaitConnection to take a server token instead of a spec.

Both solutions work but add complexity to the APIs instead of the implementation

[zxzxwu]

Can we simply add an auto_accept=True argument to create_server() method, and accept() method to channels? Thus, most flow can keep the same.

[uael]

This is a proposed alternative to #320.
It offer more control over the bumble L2CAP layer, but require more structural changes and I'm not sure which path is preferred. Although I think deferred connection accept is a good addition to bumble

[zxzxwu]

This is a proposed alternative to #320. It offer more control over the bumble L2CAP layer, but require more structural changes and I'm not sure which path is preferred. Although I think deferred connection accept is a good addition to bumble

These features make Bumble over complicated. In fact, if some scenarios require a very complicated control like timing, overwriting the entire L2CAP layer inside the usage is simpler since in Python it's very easy to achieve this.