feat: add primitive models and handler for Simplified Sliding Sync

[TheOneWithTheBraid]

WIP : No need to review so far.

This PR is part of a larger attempt to get Simplified Sliding Sync to work with the Matrix Dart SDK.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 101 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.05%. Comparing base (65cbb8d) to head (5f0486d).

Files with missing lines	Patch %	Lines
...86_simplified_sliding_sync/models/room_result.dart	0.00%	32 Missing ⚠️
...plified_sliding_sync/models/sync_request_body.dart	0.00%	13 Missing ⚠️
...ied_sliding_sync/models/sync_request_response.dart	0.00%	13 Missing ⚠️
...ified_sliding_sync/models/sliding_room_filter.dart	0.00%	10 Missing ⚠️
...msc_4186_simplified_sliding_sync/models/maybe.dart	0.00%	7 Missing ⚠️
...mplified_sliding_sync/models/sync_list_config.dart	0.00%	6 Missing ⚠️
..._simplified_sliding_sync/models/stripped_hero.dart	0.00%	5 Missing ⚠️
...ed_sliding_sync/models/required_state_request.dart	0.00%	4 Missing ⚠️
...plified_sliding_sync/models/room_subscription.dart	0.00%	4 Missing ⚠️
...186_simplified_sliding_sync/models/state_stub.dart	0.00%	4 Missing ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2198      +/-   ##
==========================================
- Coverage   57.35%   57.05%   -0.30%     
==========================================
  Files         151      162      +11     
  Lines       19292    19393     +101     
==========================================
  Hits        11064    11064              
- Misses       8228     8329     +101     

Files with missing lines	Coverage Δ
...mplified_sliding_sync/models/sync_list_result.dart	0.00% <0.00%> (ø)
...ed_sliding_sync/models/required_state_request.dart	0.00% <0.00%> (ø)
...plified_sliding_sync/models/room_subscription.dart	0.00% <0.00%> (ø)
...186_simplified_sliding_sync/models/state_stub.dart	0.00% <0.00%> (ø)
..._simplified_sliding_sync/models/stripped_hero.dart	0.00% <0.00%> (ø)
...mplified_sliding_sync/models/sync_list_config.dart	0.00% <0.00%> (ø)
...msc_4186_simplified_sliding_sync/models/maybe.dart	0.00% <0.00%> (ø)
...ified_sliding_sync/models/sliding_room_filter.dart	0.00% <0.00%> (ø)
...plified_sliding_sync/models/sync_request_body.dart	0.00% <0.00%> (ø)
...ied_sliding_sync/models/sync_request_response.dart	0.00% <0.00%> (ø)
... and 1 more

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