Allow custom operations/providers in addition to interceptors (#654)

* Allow custom operations/providers to be added in the same way custom interceptors are currently loaded

* Add new property to documentation and default yaml file

* Add test for custom operation

src/test/java/some/custom/pkg1/CustomOperationBean.java

@@ -0,0 +1,33 @@
+package some.custom.pkg1;
+
+import ca.uhn.fhir.rest.annotation.Operation;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+import org.apache.commons.io.IOUtils;
+import org.springframework.stereotype.Component;
+
+import java.io.IOException;
+
+/**
+ * Code taken from hapi documentation on how to implement an operation which handles its own request/response
+ * <a href="https://hapifhir.io/hapi-fhir/docs/server_plain/rest_operations_operations.html#manually-handing-requestresponse">...</a>
+ */
+
+@Component
+public class CustomOperationBean {
+
+	private static final org.slf4j.Logger ourLog = org.slf4j.LoggerFactory.getLogger(CustomOperationBean.class);
+
+	@Operation(name = "$springBeanOperation", manualResponse = true, manualRequest = true)
+	public void springBeanOperation(HttpServletRequest theServletRequest, HttpServletResponse theServletResponse)
+		throws IOException {
+		String contentType = theServletRequest.getContentType();
+		byte[] bytes = IOUtils.toByteArray(theServletRequest.getInputStream());
+
+		ourLog.info("Received call with content type {} and {} bytes", contentType, bytes.length);
+
+		theServletResponse.setContentType("text/plain");
+		theServletResponse.getWriter().write("springBean");
+		theServletResponse.getWriter().close();
+	}
+}

src/test/java/some/custom/pkg1/CustomOperationPojo.java

@@ -0,0 +1,28 @@
+package some.custom.pkg1;
+
+import ca.uhn.fhir.rest.annotation.Operation;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+import org.apache.commons.io.IOUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+
+public class CustomOperationPojo {
+
+	private final Logger LOGGER = LoggerFactory.getLogger(CustomOperationPojo.class);
+
+	@Operation(name = "$pojoOperation", manualResponse = true, manualRequest = true)
+	public void $pojoOperation(HttpServletRequest theServletRequest, HttpServletResponse theServletResponse)
+		throws IOException {
+		String contentType = theServletRequest.getContentType();
+		byte[] bytes = IOUtils.toByteArray(theServletRequest.getInputStream());
+
+		LOGGER.info("Received call with content type {} and {} bytes", contentType, bytes.length);
+
+		theServletResponse.setContentType("text/plain");
+		theServletResponse.getWriter().write("pojo");
+		theServletResponse.getWriter().close();
+	}
+}