Support element-level inline error messages in Unified Settings

[donnie-msft]

NuGet Product(s) Affected

Visual Studio Package Management UI

Current Behavior

Currently, when an error occurs in Unified Settings, the entire error is shown at the top of the page with a retry button, and the entire page is disabled.

Desired Behavior

Show property-specific errors beside the corresponding element.
To achieve this, Unified Settings updated their API to accept a ExternalSettingsOperationResult.InvalidValue result, which indicates the value itself is invalid and should have an error rendered with the relevant controls.

Additional Context

No response