Use OpenSSL instead of mbedTLS

[mkitti]

mbedTLS can occasionally be problematic. Could we document a procedure to use OpenSSL instead?

[nlw0]

Hi @mkitti I'm not an expert on SSL/TLS/credentials, but I suspect this might be implemented with an alternative Credentials struct, similar to the JSONCredentials we have now. Or maybe JSONCredentials could be refactored to take different types for the private_key attribute. I suspect MbedTLS really is only used in a few points of the code, but we would have to figure out how to implement the alternative.

[mkitti]

I believe there may be support in HTTP.jl already