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-test/src/main/java/org/springframework/mock/web/MockMultipartHttpServletRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -109,7 +109,7 @@ public class MockMultipartHttpServletRequest extends MockHttpServletRequest impl
 
 	@Override
 	public Map<String, MultipartFile> getFileMap() {
-		return this.multipartFiles.toSingleValueMap();
+		return this.multipartFiles.asSingleValueMap();
 	}
 
 	@Override