Remove Set(object) API from MutableJsonElement

[annelo-msft]

Description

With the move of MutableJsonDocument and related types into internal shared source, we want to limit what can be done with its APIs to prevent users from making mistakes with it. This PR removes the Set(object) method from MutableJsonElement, so people can only assign a specific set of known types to these elements. The group of supported types is essentially the same group that is supported by the JsonElement.Get APIs, with the addition of JsonElement to allow for the assignment of arbitrary JSON values.

This PR should have the side benefit of improving performance in a few ways, including removing the allocation of a JsonSerializerOptions on each creation of MutableJsonDocument, and fixing #38215.

What's in this PR

• Serialization of a non-primitive value to JSON now lives in the DynamicData layer
• We no longer cache a JsonElement with each change - instead we handle changes as the primitive type
• MutableJsonElement now has a static method that can create JsonElements from objects without taking buffers from the ArrayPool - these JsonElements are backed by JsonDocuments whose buffers will be garbage collected as normal.

Fixes #38215

[azure-sdk]

API change check

API changes are not detected in this pull request.