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-webflux/src/test/java/org/springframework/web/reactive/function/MultipartRouterFunctionIntegrationTests.java

@@ -206,7 +206,7 @@ class MultipartRouterFunctionIntegrationTests extends AbstractRouterFunctionInte
 			return request
 					.body(BodyExtractors.toMultipartData())
 					.flatMap(map -> {
-						Map<String, Part> parts = map.toSingleValueMap();
+						Map<String, Part> parts = map.asSingleValueMap();
 						try {
 							assertThat(parts).hasSize(2);
 							assertThat(((FilePart) parts.get("fooPart")).filename()).isEqualTo("foo.txt");