Support aggregation expression on fields projection.

[christophstrobl]

MongoDB allows using aggregation expressions as part of the fields projection as shown below.

db.getCollection('person').find({}, { "firstname" : 1, "lastname" : {"$toUpper" : "$lastname"}})

Initial Support:

• Pass on a given expression projection to the server as is.
• The query.fields object should be able to accept an expression eg. by using a BasicQuery.
• The @Query annotation should parse the fields attribute holding an expression.

Final Support: (might require a follow up ticket)

• Map expressions against domain types.
• Support AggregationExpression via Field projection API.