add rustls-ring feature to make ring optional

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Codecov Report

Attention: Patch coverage is 42.10526% with 11 lines in your changes missing coverage. Please review.

Project coverage is 40.43%. Comparing base (405bbe1) to head (42eef95).

Files with missing lines	Patch %	Lines
lib/g3-build-env/src/rustls.rs	0.00%	6 Missing ⚠️
lib/g3-types/src/net/rustls/cert_resolver.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #363   +/-   ##
=======================================
  Coverage   40.43%   40.43%           
=======================================
  Files        1089     1089           
  Lines       98356    98366   +10     
=======================================
+ Hits        39771    39776    +5     
- Misses      58585    58590    +5     

Flag	Coverage Δ
g3proxy	40.43% <42.10%> (+<0.01%)	⬆️
lib	25.00% <0.00%> (-0.01%)	⬇️

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