Cosmos: preserve unmapped properties

Currently unmapped properties can be overridden during updates. An entity-type level configuration like `PreserveUnmappedProperties` could be introduced to avoid this at the cost of performance.

Alternatively, https://github.com/dotnet/efcore/issues/20584 would allow to map all dynamic properties to a `Dictionary`

Start by reviewing the proposed entity-type-level PreserveUnmappedProperties configuration and the alternative in issue #20584 about mapping dynamic properties to a Dictionary. Done means updates no longer override unmapped properties, with the performance tradeoff and chosen approach clearly addressed.

Written by the indexing model from the issue text.