Support for BulkWrite into multiple collections.

[christophstrobl]

This PR introduces a new Bulk API that allows to prepare a bulk independently from MongoOperations.
The decoupling of the actual bulk to execute and the underlying operations allows for better read and maintainability.

Since multi collection bulk writes require MongoDB 8.0+ the execution layer makes sure to still route single collection bulk writes to the collection API so that users with older MongoDB Server generations are able use the new API without having to upgrade their server.

Bulk bulk = Bulk.create(builder -> builder
        .inCollection(Person.class, spec -> spec
                .insert(person1)
                .insert(person2)
                .upsert(where("_id").is("..."), new Update().set("firstname", "...")))
        .inCollection(User.class, spec -> spec.insert(new User("..."))));

BulkWriteResult result = template.bulkWrite(bulk, BulkWriteOptions.ordered());

Closes: #5087