Relax field name checks for TypedAggregations.

[christophstrobl]

Mapping field names to domain type properties should not fail if a given field name does not correspond to a property of the target domain type as it might only be part of the document stored in MongoDB.

This would allow to eg. write aggregations like

Aggregation.newAggregation(group("_id").first("name").as("name"));

without having to project("id", "name") on the fields upfront.

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Relates to: #3466