ISSUE-10438: Add a Message.getSchema() in the client API

[sijie]

Original Issue: apache#10438

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When the schema is not known at compile time, a Consumer<GenericObject> should be able to get the message schema.

Currently, a Consumer<GenericRecord> can get the AVRO message and retreive its schema, but if the message is a KeyValue for example, there is no way to get the actual Schema.

The idea is to introduce a CompletableFuture<Schema<?>> getSchema() in the Message API, returning null by default.

The implementation classes (MessageImpl, TopicMessageImpl) could then fetch the Schema if available, and eventually keep it cached as a class member.