Adding support for InsertManyOptions for insert and insertAll methods

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Adding support for InsertManyOptions for insert and insertAll methods on ReactiveMongoTemplate and MongoTemplate
Using writer on MongoTemplate.doInsertAll (which wasn't used)
Closes #4252

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[christophstrobl]

Thanks for taking the time to craft the PR. Would you please mind to remove the modifications (eg. changed imports) unrelated to the actual change. Thank you!

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Clean PR created @christophstrobl - Cleanup import was checked by mistake on commit

#4273