feat: allow multiple simultaneous active calls

[Brazol]

resolves FLU-158

🎯 Goal

Right now, our Video SDK only allows one active call at a time. We keep track of the active call in a state, and when a new call connects, we always end the previous one. We want to allow multiple active calls to happen in the same time as some clients have this requirement. This PR makes it possible.

🛠 Implementation details

• By default, only a single active call is still permitted, as it was before. To change this behavior, allowMultipleActiveCalls has been added to StreamVideoOptions.
• All places that relied on activeCall are adjusted to handle multiple active calls.
• StreamBackgroundService is also adjusted, showing multiple notifications

[codecov]

Codecov Report

Attention: Patch coverage is 18.78173% with 160 lines in your changes missing coverage. Please review.

Project coverage is 2.69%. Comparing base (fdf7a6d) to head (a88c84f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...er/lib/src/call_background/background_service.dart	0.00%	104 Missing ⚠️
packages/stream_video/lib/src/stream_video.dart	28.20%	28 Missing ⚠️
...ckages/stream_video/lib/src/core/client_state.dart	70.00%	6 Missing ⚠️
...o_flutter/lib/stream_video_flutter_background.dart	0.00%	6 Missing ⚠️
...utter/lib/stream_video_flutter_method_channel.dart	0.00%	5 Missing ⚠️
packages/stream_video/lib/src/call/call.dart	66.66%	4 Missing ⚠️
...rc/call_background/model/notification_options.dart	0.00%	4 Missing ⚠️
...ontrols/controls/toggle_screen_sharing_option.dart	0.00%	2 Missing ⚠️
...stream_video/lib/src/models/disconnect_reason.dart	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #977      +/-   ##
========================================
+ Coverage   1.75%   2.69%   +0.93%     
========================================
  Files        571     571              
  Lines      38295   38358      +63     
========================================
+ Hits         673    1032     +359     
+ Misses     37622   37326     -296     

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[renefloor]

I added some comments about backwards compatibility.

client_state.dart is not exported from the library, but is accessible through StreamVideo.state, so it's technically part of the public API.

Not sure if we can completely prevent breaking changes, but I would really prefer to start limiting them whenever possible. Especially in this case, where preventing the breaking changes also makes it easier for most developers, as having multiple active calls isn't the main usecase of the libary.