Consider switching to OpenSSL

[letmaik]

For https://github.com/microsoft/attested-fetch I needed to access the TLS certs of the connection which libcurl does not support when using mbedtls. I have a variant of the CMake script that uses OpenSSL: https://github.com/microsoft/attested-fetch/blob/main/3rdparty/openenclave-curl/CMakeLists.txt

I'm not sure if it makes sense to provide a build option to choose between mbedtls or OpenSSL but I have a feeling switching to OpenSSL as only option may make sense as well. @achamayou @herveyw-msft Any opinions?

[achamayou]

+1 for switching

[herveyw-msft]

We will eventually switch to OpenSSL but it is some time away as we have many dependencies on mbedTLS that must be abstracted out of the code. A switch would be interesting, but my first step is to just try to get this project incorporated into our build system with all its submodule dependencies.

[praenubilus]

+1 for switch or provide an option at build