Allow For Dynamic Agent Card

Given an authenticated client you might want to provide different configuration of the agent card after it has gotten the card that tells it how it authenticates (well known public card)

[Fetching Authenticated Extended Agent Card](https://a2aprotocol.ai/blog/a2a-sample-methods-and-json-responses)

The agent card could be different based upon who is talking

Currently the agent card is obtained via a non async aware method/func delegate. This does not offer any mechanism to customize the card returned if any i/o might be needed for it, unless you cause the current thread to wait, not ideal.

The TaskManager OnAgentCardQuery should instead return Task. This would even make it less clunky in A2AHttpProcessor.GetAgentCard

Additionally I do not see if there is any framework pending for an extended card after authentication. I can implement this my self in my local use of this, but didn't want to go through with it if something was planned.

[Authenticated Extended Card](https://a2aproject.github.io/A2A/latest/specification/#710-agentauthenticatedextendedcard)

Look at TaskManager.OnAgentCardQuery and A2AHttpProcessor.GetAgentCard to understand the current synchronous delegate. Change the return type to Task to allow async I/O. Check if there's existing framework for extended cards after authentication; if not, implement the authenticated extended card spec. Test with the A2A sample methods to ensure dynamic card configuration works.

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