[Blazor] Support @key at the render boundary level for declarative persistent component state

[javiercn]

If you apply @key and @rendermode into a component that uses declarative persistent component state the state fails to persist as an SSRRenderModeBoundary is introduced in the middle of the hierarchy and the @key is applied on the ancestor of the SSRRenderModeBoundary.

• We need to move the logic for finding the key to the Renderer itself (that way EndpointHtmlRenderer can adjust the behavior).
• The @key needs to be included in the marker we emit and made available in the .NET side so that the right state key can be derived.