Provide accessors in MongoItemWriter to facilitate extensions

[vincendep]

Expected Behavior

Provide accessors for collection and delete properties of MongoItemWriter, so that would be easier extending the class

Current Behavior

MongoItemWriter doesn't provide accessors for collection and delete properties

Context

Currently the MongoItemWriter has an extension point in doWrite method. I encountered some difficulties implementing
a custom "write or update" logic respecting the original contract, due to the fact that some private properties (collection and delete) don't provide an accessor method.

[santfirax]

Hello @vincendep. I'm wondering what exactly is what you need. Do you need a getter for those two fields or would you like to get protected fields for extending the class?.

[santfirax]

Hello @vincendep I have added a PR for this issue. @benas any chance that you can take a look at the PR? #3975

[vincendep]

Hello @santfirax. I don't know what is the best way to go, I just found a bit difficult to extend the MongoItemWriter. I thought about getters only because the one for the MongoTemplate is already provided

[tolpp]

Hi, I also have added PR ( #4148 ) for this issue. Similar discussion has been made for templete property in #1037, and resolved with PR #447 with commit cae2cab. I did similar changes for this issue.