fix(session): fix encoding, use tg.Config subset

[ernado]

The tg.Config cannot be marshalled from json now,
fixing it and reducing it to subset.

This should be backward compatible.

Fix #885

[codecov]

Codecov Report

Merging #886 (f2ce9e7) into main (f8b8bd8) will increase coverage by 0.03%.
The diff coverage is 61.11%.

@@            Coverage Diff             @@
##             main     #886      +/-   ##
==========================================
+ Coverage   69.73%   69.76%   +0.03%     
==========================================
  Files         444      444              
  Lines       21172    21174       +2     
==========================================
+ Hits        14765    14773       +8     
+ Misses       5470     5468       -2     
+ Partials      937      933       -4     

Impacted Files	Coverage Δ
session/session.go	48.27% <50.00%> (+1.60%)	⬆️
session/tdesktop.go	70.76% <100.00%> (-8.36%)	⬇️
telegram/session.go	76.92% <100.00%> (ø)
internal/mtproto/salt.go	73.33% <0.00%> (-13.34%)	⬇️
internal/mtproto/write.go	83.78% <0.00%> (-8.11%)	⬇️
internal/rpc/engine.go	77.40% <0.00%> (+2.82%)	⬆️
tgtest/handle.go	40.19% <0.00%> (+2.94%)	⬆️
internal/mtproto/conn.go	95.55% <0.00%> (+3.33%)	⬆️
tgtest/server.go	83.05% <0.00%> (+3.38%)	⬆️
internal/mtproto/read.go	61.22% <0.00%> (+4.76%)	⬆️
... and 2 more

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