String encoding for any MIME type

CharSequenceEncoder now supports all MIME types, however since encoding
Flux<String> can overlap with other encoders (e.g. SSE) there are now
two ways to create a CharSequenceEncoder -- with support for text/plain
only or with support for any MIME type.

In WebFlux configuration we insert one CharSequenceEncoder for
text/plain (as we have so far) and a second instance with support for
any MIME type at the very end.

Issue: SPR-15374

spring-core/src/test/java/org/springframework/core/codec/CharSequenceEncoderTests.java

@@ -43,7 +43,7 @@ public class CharSequenceEncoderTests extends AbstractDataBufferAllocatingTestCa
 
 	@Before
 	public void createEncoder() {
-		this.encoder = new CharSequenceEncoder();
+		this.encoder = CharSequenceEncoder.textPlainOnly();
 	}
 
 	@Test