feat(server): support tdesktop

[ernado]

Initial support for tdesktop key exchange.

Fix #829

[codecov]

Codecov Report

Merging #851 (1d5ccae) into main (d28a749) will decrease coverage by 0.08%.
The diff coverage is 50.00%.

❗ Current head 1d5ccae differs from pull request most recent head c4486d1. Consider uploading reports for the commit c4486d1 to get more accurate results

@@            Coverage Diff             @@
##             main     #851      +/-   ##
==========================================
- Coverage   69.83%   69.75%   -0.09%     
==========================================
  Files         444      444              
  Lines       21119    21163      +44     
==========================================
+ Hits        14749    14762      +13     
- Misses       5438     5466      +28     
- Partials      932      935       +3     

Impacted Files	Coverage Δ
internal/exchange/server_flow.go	47.87% <50.00%> (-2.13%)	⬇️
telegram/updates/gap_buffer.go	71.42% <0.00%> (-10.72%)	⬇️
tgtest/server.go	83.05% <0.00%> (-5.09%)	⬇️
telegram/updates/update.go	66.66% <0.00%> (-4.77%)	⬇️
telegram/updates/sequence_box.go	80.53% <0.00%> (-0.89%)	⬇️
internal/mtproto/conn.go	95.55% <0.00%> (+3.33%)	⬆️

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