Feature Request: add support for TLS PSK (pre-shared key)

[denglerchr]

It looks like MbedTLS supports the use of pre-shared id-key pairs, at least I found this example implementation in C: "https://github.com/dimakuv/mbedtls-psk-example/tree/master"
I did not find a way to use TLS PSKtogether with HTTP.jl, as it seems no functions to set this up are exported by this module (or I did not find them). 'MbedTKS.SSLConfig' seems to work only with certificates so far.

TLS PSK would be useful for our company where we have services all coming from the same company, that are communicating with each other. This would allow us to get rid of certificates, which are a bit annoying sometimes. If it fits in the roadmap, could you please add a way to add pre shared keys to an SSLConfig?