Various MultiValueMap improvements

This commit makes several improvements to MultiValueMap:
- asSingleValueMap offers a single-value view (as opposed to the
  existing toSingleValueMap, which offers a copy)
- fromSingleValue is a static method that adapts a Map<?,?> to the
  MultiValueMap interface
- fromMultiValue is a static method that adapts a Map<?,List<?>> to the
  MultiValueMap interface

Closes gh-32832

spring-webmvc/src/test/java/org/springframework/web/servlet/view/RedirectViewTests.java

@@ -146,7 +146,7 @@ class RedirectViewTests {
 		assertThat(response.getHeader("Location")).isEqualTo("https://url.somewhere.com/path?id=1");
 
 		assertThat(flashMap.getTargetRequestPath()).isEqualTo("/path");
-		assertThat(flashMap.getTargetRequestParams().toSingleValueMap()).isEqualTo(model);
+		assertThat(flashMap.getTargetRequestParams().asSingleValueMap()).isEqualTo(model);
 	}
 
 	@Test