feat!: Add server

[mzaniolo]

This PR adds a iec104 server. The api is still low level, requires knowing some details of the protocol and validating the input but I'll leave api improvements for another PR

Closes: #4

[mzaniolo]

@fluxlinkage, since you have been working with iec104 servers I would appreciate a feedback on this implementation, specially on the server interfaces and it's capabilities

[fluxlinkage]

@fluxlinkage, since you have been working with iec104 servers I would appreciate a feedback on this implementation, specially on the server interfaces and it's capabilities

Sorry for my delayed reply due to Chinese New Year.
Glad to see a Rust IEC104 server!
There is one little problem. Some commands (e.g. global interrogation) need replys (only on the same connection). Currently, it seems that it is not possible to handle this within on_new_objects in ServerCallback.
My fork (https://github.com/fluxlinkage/iec104) used a workaround by let on_new_objects return response ASDUs and send them (if any) (fluxlinkage@2fc6a42). But this introduced another problem: a lot of ASDUs may be sent as responses to a single command (e.g. global interrogation), and the number may exceed k. So a send queue should be used to avoid error (fluxlinkage@b85656e).
For reference only.

[mzaniolo]

@fluxlinkage thanks for the feedback.
I added a queue on the handler to buffer the responses as you suggested. You can find it here. It's used to enqueue asdus and dequeue.
The ServerCallback was always meant to be a low level interface. Since it's called from the main loop you would ideally just do a simple handling of the asdu and send anything over a channel for a more complex logic. I also added the rtu_server. This one is a high level interface and does most of the heavy lifting. The rtu_server interfaces should be enough for most of the user but let me know if you feel that something is missing. The rtu_server also serves as an example of how someone can build a server using the ServerCallback interface.

I ran some tests on it but I would appreciate if someone else would try out and do some testing too

[fluxlinkage]

Great work!!!

[fluxlinkage]

Broadcast address compatibility for interrogations.