Add CitationEntity & citation extension methods

[corinagum]

Port CitationEntity and supporting types from main, adapted for next/core.

• Adds AddCitation, AddAIGenerated, and AddFeedback methods to TeamsActivity
• Enables OMessage sub-dispatch in EntityList.FromJsonArray() with @type routing ("Message" --> CitationEntity, "CreativeWork" --> SensitiveUsageEntity)
• Adds typed FeedbackLoopEnabled property to TeamsChannelData
• Registers new citation types in TeamsActivityJsonContext: (CitationEntity, CitationClaim, CitationAppearanceDocument, CitationImageObject, CitationAppearance)
• Registers previously missing entity types in TeamsActivityJsonContext: (OMessageEntity, SensitiveUsageEntity, DefinedTerm, ProductInfoEntity, StreamInfoEntity)
• Unit tests

Notes

• Not porting UpdateEntity(IEntity, IEntity); AddCitation does remove + add inline
• Samples.AI/CitationsHandler not ported
• IMessageEntity not needed - concrete classes only in 2.1
• IMessageJsonConverter replaced by DeserializeMessageEntity in EntityList

[rido-min]

Changes are looking good !!

I would add some sample code, maybe in TeamsBot to show citations in action.

and a few notes in the comments

[rido-min]

• No samples use core namespaces yet

We have samples for Core, Compat and new FrontEnd in core/samples