[.Net] fix #2695 and #2884

[LittleLittleCloud]

Why are these changes needed?

This PR implements the IOrchestrator pattern mentioned in #2695 which allows user to easily modify the orchestrator used in group chat. It also adds the following three orchestrators

• round_robin
• role_play
• workflow

This PR also fix #2884 which is caused by continuous roles in chat history and is a pattern that is not allowed in anthropic model

Related issue number

fix #2695

Checks

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• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.