Provide autoconfiguration to enable TLS for Web Server using PEM files instead of Keystore

[making]

It is now possible to make your web server TLS enabled using properties such as server.ssl.key-store.
However, it only supports Keystore and does not support the widely used PEM format.
In Kubernetes, it is common to store TLS certs as PEM files in Secret. cert-manager, which is popular for issuing certificates, also creates a Secret in PEM format.
If my understanding is correct, in order to enable TLS using this PEM file in a Spring Boot application, you need to convert it to a jks file using keytool. In Kubernetes initContainer to mount the TLS PEM file from Secret and perform this conversion process is required, which is very painful.
As far as I can simply find out, Tomcat and Netty (maybe others) can pass PEM files directly in addition to the KeyStore to enable TLS as follows.

	// Netty
	@Bean
	public NettyServerCustomizer customizer() {
		return httpServer -> httpServer.secure(sslContextSpec -> {
			Http11SslContextSpec spec = Http11SslContextSpec.forServer(new File("<path to server.crt>"), new File("<path to server.key>"));
			sslContextSpec.sslContext(spec);
		});
	}

	// Tomcat
	@Bean
	public TomcatConnectorCustomizer customizer() {
		return connector -> {
			AbstractHttp11JsseProtocol<?> protocol = (AbstractHttp11JsseProtocol) connector.getProtocolHandler();
			protocol.setSSLEnabled(true);
			protocol.setSSLCertificateFile("<path to server.crt>");
			protocol.setSSLCertificateKeyFile("<path to server.key>");
		};
	}

It tested above code with self-signed certificates generated as follows

mkdir certs
curl -sL https://gist.github.com/making/92dc4c3ab7ee7be8a31f5f8345c6df88/raw/fa6f6ef52b74af7af20a2ac8cd921a98f5650a91/generate-certs.sh > certs/generate-certs.sh 
docker run --rm -v ${PWD}/certs:/certs hitch bash /certs/generate-certs.sh 127-0-0-1.sslip.io

It would be very convenient if this was provided in the auto configuration.

(Tip: cert-manager uses PKCS#1 format by default. It won't work unless you set PKCS#8 in certificate.spec.privateKey.encoding)

related issue #24940

[philwebb]

We'll have to also look at Jetty and Undertow. Regardless, I think this would be a nice enhancement.