.Net Agents - Mark experimental features

[crickman]

As part of agent graduation, move the experimental declaration from the project level to specific features that are not graduating:

• AgentChat/AgentGroupChat: The existing multi-agent chat.
• AgentChannel: The channel construct only exists to enable multi-agent chat.
• AggregatorAgent: In its current form, this agent is intrinsically bound to AgentChat.
• AgentChatSerializer: This exists as an outgrowth of AgentChat and is not conducive to distributed patterns.
• Agent: The base abstraction defines three methods that are not associated with patterns being graduated:
  1. GetChannelKeys
  2. CreateChannelAsync
  3. RestoreChannelAsync

[shethaadit]

Hi @crickman / @markwallace-microsoft, I will raise PR for this soon. :)

[crickman]

Hi @crickman / @markwallace-microsoft, I will raise PR for this soon. :)

Thanks, I've got it covered: #10600