OGM-293 OgmCustomLoader

I've created an OgmCustomLoader that should allow the execution of native queries.
Right now I've implemented it only on mongodb and it doesn't support parameters in the quey.

I should probably create a separate issue for this but I wanted to ask some feedback before.

I've decided to keep it separated from the one that @gunnarmorling did (https://github.com/hibernate/hibernate-ogm/pull/218) since we didn't include it yet. We are working on two separate aspects anyway and it should not be difficult to merge the two patches eventually.

Cheers,
Davide