Rust: RegistrationConfig and CredentialConfig types

[youyuanwu]

Description

Provide safe Rust wrapper for RegistrationConfig and CredentialConfig types.
Removed the manual ffi code respectively.
Replaced the crate feature switch schannel with openssl. This makes the rust code by default compiles with schannel on winodws unless openssl feature is enabled. For linux, openssl is the default.

Testing

Added tests for loading credentials for each credential types.

Documentation

NA

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.13%. Comparing base (ba9e8be) to head (ddc8e91).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4881      +/-   ##
==========================================
- Coverage   87.30%   87.13%   -0.17%     
==========================================
  Files          56       56              
  Lines       17630    17685      +55     
==========================================
+ Hits        15391    15410      +19     
- Misses       2239     2275      +36     

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