Add getters for collection and mode in MongoItemWriter

[santfirax]

• Change access modifiers for collection and delete fields from private to protected in order to be access those when extending a class.
• Add Getter for collection and delete field in order to access those two fields.

[fmbenhassine]

Rebased and merged as 5e51547. Please note that the delete flag was replaced with an enum in #4351. Therefore, I updated the getter accordingly. Moreover, since we provide getters now, there is no need to make the fields protected (to be consistent with the template field).

Thank you for your contribution!