Add endpoint to expose public key for JWT verification

The dynamic log level feature uses JWTs. These can be verified by a public key.
Since the sample app can generate those JWTs this new endpoint exposes the
public key required to verify the generated tokens.

Author: KarstenSchnitter

sample-spring-boot/src/main/java/com/sap/hcp/cf/logging/sample/springboot/controller/PublicKeyController.java

@@ -0,0 +1,38 @@
+package com.sap.hcp.cf.logging.sample.springboot.controller;
+
+import java.security.interfaces.RSAPublicKey;
+import java.util.Base64;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import com.sap.hcp.cf.logging.sample.springboot.keystore.TokenKeyProvider;
+
+/**
+ * This controller provides an endpoint to get the public key used for signing
+ * the tokens generated by the {@link TokenController}.
+ */
+@RestController
+public class PublicKeyController {
+
+	private TokenKeyProvider keyProvider;
+
+	public PublicKeyController(@Autowired TokenKeyProvider keyProvider) {
+		this.keyProvider = keyProvider;
+	}
+
+	/**
+	 * Returns the public key in Base64 encoding used to sign the JWTs created by
+	 * the {@link TokenController}.
+	 * 
+	 * @return
+	 */
+	@GetMapping("/publickey")
+	public String getPublicKey() {
+		String keyId = keyProvider.getPrivateKeyId();
+		RSAPublicKey publicKey = keyProvider.getPublicKeyById(keyId);
+		return Base64.getEncoder().encodeToString(publicKey.getEncoded());
+	}
+
+}

sample-spring-boot/src/main/java/com/sap/hcp/cf/logging/sample/springboot/controller/TokenController.java

@@ -16,7 +16,7 @@
 import com.sap.hcp.cf.logging.sample.springboot.service.TokenGenerator;
 
 /**
- * This controller provides and endpoint to create new JWT tokens. These token
+ * This controller provides an endpoint to create new JWT tokens. These token
  * can be used as headers of HTTP request to dynamically switch the log level.
  */
 @RestController