Add support for MongoDB Collation

It seems there is no easy way to perform searches that ignore accents on strings.

MongoDB has a feature called Collation that supports this, but it seems that GORM for MongoDB does not support it: https://docs.mongodb.com/manual/reference/collation/

I think it would be a good improvement, specially for those of us that use languages with accents :-)

Start by reviewing the MongoDB Collation documentation linked in the issue and locating GORM for MongoDB's query and configuration entry points. Determine how collation should be exposed for searches and how it interacts with accent-insensitive matching. Done means GORM for MongoDB can use MongoDB collation for the requested searches, with coverage for the supported behavior.

Written by the indexing model from the issue text.