Added Option.Marshal to support Avro serialization

[nicfit]

For example:

var opts = slogkafka.Option{
    Level:       level,
    KafkaWriter: &writer,
    Marshal: func(v any) ([]byte, error) {
        return avro.Marshal(schema, v)
    },
}

[samber]

i'm just going to rename Marshal to Marshaler, to match the naming of other slog handlers of my collection

thanks for the contrib!

[samber]

-> v2.6.0