Make field names configurable

[N4SoftwareNinja]

Hello together,

we currently have an use case where we would have to change the preconfigured field names in AbstractMongoSessionConverter/JdkMongoSessionConverter and found that this isn't possible without copying a lot of spring-data-mongodb classes.
So we are curious if there is a reason why does field names where not made configurable, especially since it would simply be a matter of switching them from constants to normal class member variables and providing a setter since everything else in those classes already works with those class constants.

If you would be fine with this change I also could provide a Pull Request for this issue.

Greetings