Improve BaseFhirJsonDeserializer seeking for resourceType #3295
Description
We need to know resourceType to create a mapping as that will determine behavior for each property.
Right now we do it via seeking for it, the 'rewinding' back, but that will break in network streamed scenarios.
Also worst case scenario, we will get Bundle with a lot of entries, but resourceType appears at the very end of a Bundle, we will read whole resource, then rewind back, then read it again.
Might be a nice improvement to pre-read the properties into a (propertyName, JsonElement) pairs until we encounter the resourceType, then process those, then continue reading the json.
Hopefully a lot of logic could be left as is by wrapping Utf8JsonReader into an API compatible ref struct that will first read over cache, then over remaining data.