Add coroutine variant of WebExceptionHandler

See gh-32931

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

@@ -0,0 +1,15 @@
+package org.springframework.web.server
+
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.reactor.mono
+import reactor.core.publisher.Mono
+import kotlin.coroutines.CoroutineContext
+
+abstract class CoWebExceptionHandler : WebExceptionHandler {
+	final override fun handle(exchange: ServerWebExchange, ex: Throwable): Mono<Void> {
+		val context = exchange.attributes[CoWebFilter.COROUTINE_CONTEXT_ATTRIBUTE] as CoroutineContext?
+		return mono(context ?: Dispatchers.Unconfined) { coHandle(exchange, ex) }.then()
+	}
+
+	protected abstract suspend fun coHandle(exchange: ServerWebExchange, ex: Throwable)
+}