Enable support for adding KeyManager

[Hakky54]

Subsystem
Client, CIO module

Is your feature request related to a problem? Please describe.
I am currently configuring the client to communicate over mutual tls/mutual authentication. The server will require the client to identify itself. To enable this feature a client needs a sslcontext with a preconfigured keymanager initialized. With the Apache or OkHttp module this is already possible with ktor, because it accepts a custom sslcontext or sslsocketfactory, see below:

For Apache HttpClient

import io.ktor.client.HttpClient
import io.ktor.client.engine.apache.Apache

class ApacheExampleClient {

    val client = HttpClient(Apache) {
        engine {
            sslContext = //custom sslcontext
        }
    }

}

For OkHttp

import io.ktor.client.HttpClient
import io.ktor.client.engine.okhttp.OkHttp

class OkHttpExampleClient {

    val client = HttpClient(OkHttp) {
        engine {
            config {
                sslSocketFactory(socketFactory, trustManager)
                hostnameVerifier(hostnameVerifier)
            }
        }
    }

}

The CIO module accepts couple of https parameters, such as trustmanager and secure random. I am assuming that the CIO module will use these values to create a custom sslcontext. With the current options available in the TLSConfigBuilder, which is being used by the CIOEngineConfig, it is not possible to provide the keymaterial.

Describe the solution you'd like
I would like to have the possibility to also provide a keymanager within the TLSConfigBuilder. In that way the client can identify itself when the server is requesting the client to provide client-certificate during the handshake process.

With KeyManager and TrustManager

import io.ktor.client.HttpClient
import io.ktor.client.engine.cio.CIO

class CioExampleClient {

    val client = HttpClient(CIO) {
        engine {
            https {
                keyManager = //keymanager
                trustManager = //trustmanager
            }
        }
    }

}

Motivation to include to ktor
This option will enable the end-user to get the same behaviour as other clients like apache and okhttp for mutual authentication. I am not quite sure if it is easy to implement or even possible, but it would be a cool feature to have it.

[oleg-larshin]

Please check the following ticket on YouTrack for follow-ups to this issue. GitHub issues will be closed in the coming weeks.