Fetch DC203 from Telegram

[9seconds]

This PR reflects latest changes in Telegram: now it publishes CDN DC in middle proxy public list. This PR invalidates dc-overrides configuration setting and establishes fetching using Telegram itself.

Also, there is a refactoring for the obfuscation package

[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.21951% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.21%. Comparing base (45ce5c2) to head (bb320e9).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
mtglib/proxy.go	41.66%	21 Missing ⚠️
mtglib/internal/dc/public_config_updater.go	71.73%	8 Missing and 5 partials ⚠️
mtglib/internal/obfuscation/handshake_frame.go	66.66%	8 Missing and 3 partials ⚠️
mtglib/internal/obfuscation/obfuscator.go	86.04%	3 Missing and 3 partials ⚠️
mtglib/internal/dc/telegram.go	33.33%	2 Missing ⚠️
mtglib/internal/faketls/conn.go	71.42%	1 Missing and 1 partial ⚠️
mtglib/internal/obfuscation/conn.go	77.77%	1 Missing and 1 partial ⚠️
mtglib/internal/dc/addr.go	0.00%	1 Missing ⚠️
mtglib/internal/dc/updater.go	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
+ Coverage   68.50%   69.21%   +0.71%     
==========================================
  Files          84       81       -3     
  Lines        2715     2716       +1     
==========================================
+ Hits         1860     1880      +20     
+ Misses        757      733      -24     
- Partials       98      103       +5     

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