Add DataBufferUtils.compose

Added a utility method that composes data buffers into a single buffer.
Depending on the `DataBuffer` implementation, the returned buffer may be
a single buffer containing all data of the provided buffers, or it may
be a true composite that contains references to the buffers.

Issue: SPR-16365

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferTests.java

@@ -479,5 +479,18 @@ public class DataBufferTests extends AbstractDataBufferAllocatingTestCase {
 		release(buffer);
 	}
 
+	@Test
+	public void composite() {
+		DataBuffer composite = this.bufferFactory.compose(Arrays.asList(stringBuffer("a"),
+				stringBuffer("b"), stringBuffer("c")));
+		assertEquals(3, composite.readableByteCount());
+		byte[] bytes = new byte[3];
+		composite.read(bytes);
+
+		assertArrayEquals(new byte[] {'a','b','c'}, bytes);
+
+		release(composite);
+	}
+
 
 }

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferUtilsTests.java

@@ -351,5 +351,19 @@ public class DataBufferUtilsTests extends AbstractDataBufferAllocatingTestCase {
 		};
 	}
 
+	@Test
+	public void compose() {
+		DataBuffer foo = stringBuffer("foo");
+		DataBuffer bar = stringBuffer("bar");
+		DataBuffer baz = stringBuffer("baz");
+		Flux<DataBuffer> flux = Flux.just(foo, bar, baz);
+
+		DataBuffer result = DataBufferUtils.compose(flux).block(Duration.ofSeconds(5));
+
+		assertEquals("foobarbaz", DataBufferTestUtils.dumpString(result, StandardCharsets.UTF_8));
+
+		release(result);
+	}
+
 
 }