Quarkus REST should check for `@CustomSerialization` annotations on resource methods returning `Multi`

Describe the bug

I mentioned that custom serialization for Multi doesn't work because serialization flow goes roughly like this:

org.jboss.resteasy.reactive.server.core.StreamingUtil#serialiseEntity
io.quarkus.resteasy.reactive.jackson.runtime.serialisers.FullyFeaturedServerJacksonMessageBodyWriter#writeTo
org.jboss.resteasy.reactive.server.jackson.JacksonMessageBodyWriterUtil#doLegacyWrite

Which effectively means that custom serialization is ignored because that one would require non-default writer as when io.quarkus.resteasy.reactive.jackson.runtime.serialisers.FullyFeaturedServerJacksonMessageBodyWriter#writeResponse is called.

Expected behavior

I didn't find documented that this shouldn't work for the Multi reactive type, therefore I would expect it to work.

Actual behavior

Custom serialization is not applied.

How to Reproduce?

I have tests for this: https://github.com/quarkusio/quarkus/commit/d3212670cb47ba431648378ecc4bf7f9b12e3785 just uncomment io.quarkus.resteasy.reactive.jackson.deployment.test.ResponseType#MULTI and run mvn clean test -f extensions/resteasy-reactive/rest-jackson/deployment/ -Dtest=SecureFieldDetectionTest

Output of `uname -a` or `ver`

Fedora 38

Output of `java -version`

OpenJDK 21

Quarkus version or git rev

999-SNAPSHOT

Build tool (ie. output of `mvnw --version` or `gradlew --version`)

Apache Maven 3.9.4

Additional information

No response

quarkusio / quarkus