No example or function to create an SSLConfig where you need a cert as a client

[fchorney]

I recently had to use HTTP.jl and MbedTLS.jl to set up a situation where I was a client trying to submit a post request to an endpoint. The issue was that I needed to have a certificate to do this. There is a function the code base which sets this up when you want to act as a server (https://github.com/JuliaLang/MbedTLS.jl/blob/master/src/MbedTLS.jl#L108-L120) but not a client.

It would be nice if there was a similar function for when you want to act as a client. I ended up using the following:

    # Create SSL RNG
    entropy = MbedTLS.Entropy()
    rng = MbedTLS.CtrDrbg()
    MbedTLS.seed!(rng, entropy)

    # Read in the certificate files
    cert = MbedTLS.crt_parse_file(ssl_cert_path)
    key = MbedTLS.parse_keyfile(ssl_key_path)

    # Create SSLConfig with our pulled in certificate
    conf = MbedTLS.SSLConfig()
    MbedTLS.config_defaults!(conf)
    MbedTLS.rng!(conf, rng)
    MbedTLS.own_cert!(conf, cert, key)
    MbedTLS.ca_chain!(conf)

    return conf

On that note, similar to #231 it would be nice if MbedTLS could accept a singular pfx file instead of having to break it up into cert and key.