Adds spring-cloud-starter-bootstrap.

This allows users to opt-in to bootstrap via classpath rather than properties.

spring-cloud-starter-bootstrap/src/main/java/org/springframework/cloud/bootstrap/marker/Marker.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2013-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.bootstrap.marker;
+
+/**
+ * A marker class, so that, if present, spring cloud bootstrap is enable similar to how
+ * 'spring.cloud.bootstrap.enabled=true' works.
+ */
+public abstract class Marker {
+
+	private Marker() {
+
+	}
+
+}

spring-cloud-starter-bootstrap/src/test/java/org/springframework/cloud/bootstrap/marker/MarkerTests.java

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+/*
+ * Copyright 2013-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.bootstrap.marker;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.SpringBootConfiguration;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.core.env.Environment;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@SpringBootTest
+public class MarkerTests {
+
+	@Autowired
+	private Environment env;
+
+	@Test
+	public void testBootstrapIsEnabled() {
+		assertThat(env.getProperty("my.prop")).isEqualTo("my.value");
+	}
+
+	@SpringBootConfiguration
+	@EnableAutoConfiguration
+	protected static class TestConfig {
+
+	}
+}

spring-cloud-starter-bootstrap/src/test/resources/bootstrap.properties

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+my.prop=my.value