Improve performance on json to psobject conversion (PowerShell#7482)

This amounts so a speed-up in the order of 7x. Cmdlets that benefit from this are `Convertfrom-Json` and `Invoke-RestMethod`. There are three main changes:

- Convert from `JArray` directly to object array instead of creating a `List` and do `list.ToArray()`.
- Avoid iterating through `PSObject.Properties` to check for existing members, since that is a very slow and allocation heavy code path.
- Pre-allocate the members in `PSObject` by using the newly added constructor accepting an initial member count.