Move ServerWebExchange Kotlin extensions

This commit moves ServerWebExchange Kotlin extensions
where they belong: in the spring-web module with the
org.springframework.web.server package, like
ServerWebExchange itself.

The extensions in the wrong location are deprecated
and semi-automated migration to the new variants is
made possible via @Deprecated + ReplaceWith(...).

Some tests have been added as well.

Closes gh-31046

spring-web/src/main/kotlin/org/springframework/web/server/ServerWebExchangeExtensions.kt

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2002-2023 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.server
+
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.reactive.awaitSingle
+import kotlinx.coroutines.reactor.mono
+import org.springframework.http.codec.multipart.Part
+import org.springframework.util.MultiValueMap
+import java.security.Principal
+
+/**
+ * Coroutines variant of [ServerWebExchange.getFormData].
+ *
+ * @author Sebastien Deleuze
+ * @since 6.1
+ */
+suspend fun ServerWebExchange.awaitFormData(): MultiValueMap<String, String> =
+		this.formData.awaitSingle()
+
+/**
+ * Coroutines variant of [ServerWebExchange.getMultipartData].
+ *
+ * @author Sebastien Deleuze
+ * @since 6.1
+ */
+suspend fun ServerWebExchange.awaitMultipartData(): MultiValueMap<String, Part> =
+		this.multipartData.awaitSingle()
+
+/**
+ * Coroutines variant of [ServerWebExchange.getPrincipal].
+ *
+ * @author Sebastien Deleuze
+ * @since 6.1
+ */
+suspend fun <T : Principal> ServerWebExchange.awaitPrincipal(): T =
+		this.getPrincipal<T>().awaitSingle()
+
+/**
+ * Coroutines variant of [ServerWebExchange.getSession].
+ *
+ * @author Sebastien Deleuze
+ * @since 6.1
+ */
+suspend fun ServerWebExchange.awaitSession(): WebSession =
+		this.session.awaitSingle()
+
+/**
+ * Coroutines variant of [ServerWebExchange.Builder.principal].
+ *
+ * @author Sebastien Deleuze
+ * @since 6.1
+ */
+fun ServerWebExchange.Builder.principal(supplier: suspend () -> Principal): ServerWebExchange.Builder
+        = principal(mono(Dispatchers.Unconfined) { supplier.invoke() })