GH-265 Added support for injecting execution context

- Added support for injecting target execution context into the ApplicationContext in AbstractSpringFunctionAdapterInitializer, effectively making it available to all adapters (if one is provided)
- Added test, docs

Resolves #265

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/AbstractSpringFunctionAdapterInitializer.java

@@ -42,7 +42,9 @@ import org.springframework.boot.SpringApplication;
 import org.springframework.boot.WebApplicationType;
 import org.springframework.cloud.function.context.catalog.FunctionInspector;
 import org.springframework.cloud.function.context.config.FunctionContextUtils;
+import org.springframework.context.ApplicationContextInitializer;
 import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.support.GenericApplicationContext;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -57,6 +59,11 @@ public abstract class AbstractSpringFunctionAdapterInitializer<C> implements Clo
 
 	private static Log logger = LogFactory.getLog(AbstractSpringFunctionAdapterInitializer.class);
 
+	/**
+	 * Name of the bean for registering the target execution context passed to `initialize(context)` operation.
+	 */
+	public static final String TARGET_EXECUTION_CTX_BEAN_NAME = "targetExecutionContext";
+
 	private final Class<?> configurationClass;
 
 	private Function<Publisher<?>, Publisher<?>> function;
@@ -102,6 +109,8 @@ public abstract class AbstractSpringFunctionAdapterInitializer<C> implements Clo
 		}
 		logger.info("Initializing: " + this.configurationClass);
 		SpringApplication builder = springApplication();
+
+		this.registerTargetContext(targetContext, builder);
 		ConfigurableApplicationContext context = builder.run();
 		context.getAutowireCapableBeanFactory().autowireBean(this);
 		this.context = context;
@@ -113,6 +122,19 @@ public abstract class AbstractSpringFunctionAdapterInitializer<C> implements Clo
 		}
 	}
 
+	private void registerTargetContext(C targetContext, SpringApplication builder) {
+		if (targetContext != null) {
+			builder.addInitializers(new ApplicationContextInitializer<ConfigurableApplicationContext>() {
+				@SuppressWarnings("unchecked")
+				@Override
+				public void initialize(ConfigurableApplicationContext applicationContext) {
+					((GenericApplicationContext) applicationContext).registerBean(TARGET_EXECUTION_CTX_BEAN_NAME,
+							(Class<C>) targetContext.getClass(), (Supplier<C>) () -> targetContext);
+				}
+			});
+		}
+	}
+
 	protected FunctionInspector getInspector() {
 		return inspector;
 	}