Simplify usage of custom ssl configuration

[Hakky54]

This PR is a followup of the following earlier PR #673 Although that pull request didn't get merged, the code changes has been comitted to the main branch by the main developer, see here for the specific commit: b0df981

Context
With the earlier commit it is now possible to programatically configure the ssl configuration of tomcat instead of using properties and delegating to tomcat to construct the ssl configuration. This opens the possibility of reloading the ssl configuration or other customizations as shown also here: sslcontext-kickstart

Problem statement
Boilerplate code is needed by the end-user to provide a custom ssl configuration. Tomcat takes a custom SSLContext, the full name is org.apache.tomcat.util.net.SSLContext while the end-user has javax.net.ssl.SSLContext. So the end-user is required to create an implementation of org.apache.tomcat.util.net.SSLContext which acts as a wrapper. This sslcontext needs to be passed to SSLHostConfigCertificate to further configure the server.

Solution
Provide a helper class which acts as a wrapper to reduce the boilerplate code. The utility interface is able to provide a method to wrap the required objects, in this case javax.net.ssl.SSLContext, KeyManager, TrustManager in a org.apache.tomcat.util.net.SSLContext

Example usage
Below is an example usage with Spring Boot and Tomcat

import org.apache.catalina.connector.Connector;
import org.apache.coyote.http11.AbstractHttp11Protocol;
import org.apache.tomcat.util.net.SSLHostConfig;
import org.apache.tomcat.util.net.SSLHostConfigCertificate;
import org.apache.tomcat.util.net.SSLHostConfigCertificate.Type;
import org.apache.tomcat.util.net.SSLUtil;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.web.embedded.tomcat.TomcatConnectorCustomizer;
import org.springframework.context.annotation.Configuration;

import javax.net.ssl.SSLContext;
import javax.net.ssl.X509KeyManager;
import javax.net.ssl.X509TrustManager;

@Configuration
public class SSLConnectorCustomizer implements TomcatConnectorCustomizer {

    private final int port;

    public SSLConnectorCustomizer(@Value("${server.port}") int port) {
        this.port = port;
    }

    @Override
    public void customize(Connector connector) {
        X509KeyManager keyManager = ...;        // initialized keyManager
        X509TrustManager trustManager = ...;    // initialized trustManager
        SSLContext sslContext = ...;            // initialized sslContext

        connector.setScheme("https");
        connector.setSecure(true);
        connector.setPort(port);

        AbstractHttp11Protocol<?> protocol = (AbstractHttp11Protocol<?>) connector.getProtocolHandler();
        protocol.setSSLEnabled(true);

        org.apache.tomcat.util.net.SSLContext context = SSLUtil.createSSLContext(sslContext, keyManager, trustManager);
        SSLHostConfig sslHostConfig = new SSLHostConfig();
        SSLHostConfigCertificate certificate = new SSLHostConfigCertificate(sslHostConfig, Type.UNDEFINED);
        certificate.setSslContext(context);
        sslHostConfig.addCertificate(certificate);
        protocol.addSslHostConfig(sslHostConfig);
    }

}

In the past I created the same PR, but I assumed it would not get merged and therefor I gave up and closed the PR. But I still think it is useful and decided the recreate the PR to give it another shot. Looking forward to your feedback and decision for this PR.