CDN redirect support and retry handling

[borzovplus]

Added "AllowCDN" flag to telegram client.

Added "WithRetryHandler" for download builder.

[codecov]

Codecov Report

❌ Patch coverage is 80.63466% with 238 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.67%. Comparing base (f37cb63) to head (0e7aeae).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
telegram/downloader/cdn_state_machine.go	74.05%	37 Missing and 18 partials ⚠️
telegram/downloader/cdn_verify.go	77.22%	32 Missing and 14 partials ⚠️
telegram/cdn_pool_manager.go	80.58%	20 Missing and 13 partials ⚠️
telegram/cdn.go	86.00%	14 Missing and 7 partials ⚠️
telegram/pool.go	70.83%	10 Missing and 4 partials ⚠️
telegram/downloader/retry.go	71.05%	8 Missing and 3 partials ⚠️
telegram/downloader/builder.go	87.67%	7 Missing and 2 partials ⚠️
telegram/cdn_conn_dead.go	76.66%	5 Missing and 2 partials ⚠️
telegram/dcs/plain.go	0.00%	7 Missing ⚠️
telegram/dc_transfer.go	0.00%	6 Missing ⚠️
... and 11 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1695      +/-   ##
==========================================
+ Coverage   68.63%   69.67%   +1.04%     
==========================================
  Files         452      461       +9     
  Lines       18910    20043    +1133     
==========================================
+ Hits        12978    13965     +987     
- Misses       4963     5042      +79     
- Partials      969     1036      +67     

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