Improve conditional requests support

Prior to this commit, Spring MVC and Spring WebFlux would not support
conditional requests with `If-Match` preconditions. As underlined in the
RFC9110 Section 13.1, those are related to the `If-None-Match`
conditions, but this time only performing requests if the resource
matches the given ETag.

This feature, and in general the `"*"` request Etag, are generally
useful to prevent "lost updates" when performing a POST/PUT request: we
want to ensure that we're updating a version with a known version or
create a new resource only if it doesn't exist already.

This commit adds `If-Match` conditional requests support and ensures
that both `If-Match` and `If-None-Match` work well with `"*"` request
ETags.

We can't rely on `checkNotModified(null)`, as the compiler can't decide
between method variants accepting an ETag `String` or a Last Modified
`long`. Instead, developers should use empty ETags `""` to signal that
no resource is known on the server side.

Closes gh-24881

spring-web/src/testFixtures/java/org/springframework/web/testfixture/http/server/reactive/MockServerHttpRequest.java

@@ -360,6 +360,12 @@ public final class MockServerHttpRequest extends AbstractServerHttpRequest {
 		 */
 		B ifUnmodifiedSince(long ifUnmodifiedSince);
 
+		/**
+		 * Set the values of the {@code If-Match} header.
+		 * @param ifMatches the new value of the header
+		 */
+		B ifMatch(String... ifMatches);
+
 		/**
 		 * Set the values of the {@code If-None-Match} header.
 		 * @param ifNoneMatches the new value of the header
@@ -556,6 +562,12 @@ public final class MockServerHttpRequest extends AbstractServerHttpRequest {
 			return this;
 		}
 
+		@Override
+		public BodyBuilder ifMatch(String... ifMatches) {
+			this.headers.setIfMatch(Arrays.asList(ifMatches));
+			return this;
+		}
+
 		@Override
 		public BodyBuilder ifNoneMatch(String... ifNoneMatches) {
 			this.headers.setIfNoneMatch(Arrays.asList(ifNoneMatches));