In order to avoid OpenSSL dependency (#10763) in libgit2, custom TLS stream framework should be utilized to construct streams with different TLS library.

Required:

• Native TLS stream implementation based on MbedTLS.jl: https://github.com/wildart/MbedTLSStream.jl
• remove OpenSSL dependency while building libgit2 for Linux
  • -DUSE_OPENSSL=OFF
  • Enable https transport for custom TLS streams in libgit2, see Enable https transport for custom TLS streams libgit2/libgit2#3850
• Initialization of custom TLS stream in LibGit2 module
• MbedTLS.jl should use on libmbed* from julia build, see Use mbedTLS binaries shipped with Julia 0.5 MbedTLS.jl#54
• Proxy support

Refs: #17246, https://github.com/JuliaWeb/MbedTLS.jl