Support custom KeyStore and TrustStore providers

[jalaziz]

With dynamic CA infrastructure, it's not necessarily true that a certificate or keystore will be available statically on the machine.

For example, it's possible to use Vault to dynamically create a key pair and create a KeyStore at runtime.

It would be great if we could supply custom "providers" that allow loading the KeyStore and TrustStore dynamically.

While it's possible to use the library directly without the config, you lose out on the extra safety checks provided by the ConfigSSLContextBuilder class. Also, it's not possible to use the API directly where the config is expected to be used (e.g. akka-remoting).