Cosmos: Modernize our JSON serialization/deserialization

[roji]

Our Cosmos deserialization is currently based on using Newtonsoft.Json to deserialize to DOM JTokens, which are then e.g. processed by our shapers. We should take a look at the entire approach here and decide what to do; at the very least, we should consider moving from Newtonsoft.Json to System.Text.Json - using Newtonsoft.Json notably means that deserialization happens synchronously (its JsonSerializer doesn't support async).

To improve performance, we could go all the way and stop serializing/deserializing to DOM types altogether, and use low-level parsing directly from UTF8, like we already do in relational.

[AndriySvyryd]

To improve performance, we could go all the way and stop serializing/deserializing to DOM types altogether, and use low-level parsing directly from UTF8, like we already do in relational.

Yes and #14570