Enable A2A Agents as workers in manager/workers pattern

[sathvikbhagavan]

This PR enables A2AAgent to be used as workers in ManagerWorkers.

For A2AAgent to be used in ManagerWorkers as a worker:

1. Types of compatible workers needed to be changed
2. _validate_agent_unicity needed to refactored as manager agent can only be Agent while workers can be Agent, ManagerWorkers and A2AAgent.
3. Agentspec deserialization also needed a refactor to account for this.

Also, this PR adds ManagerWorkers section in the howto guide for using A2AAgents.

The how to guide is restructured to list all the ways A2AAgent can be used. It can be extended to flows when A2AAgent supports it.

[nasseur]

Internal regression failed: Build ID #510

[nasseur]

Internal regression failed: Build ID #511

[nasseur]

Internal regression failed: Build ID #512

[nasseur]

Internal regression failed: Build ID #513

[nasseur]

Internal regression succeeded 🍏: Build ID #525

[nasseur]

Internal regression succeeded 🍏: Build ID #527

[nasseur]

Internal regression succeeded 🍏: Build ID #535

[dhilloulinoracle]

@sathvikbhagavan , thanks a lot!
Please add a bit more info in the PR description of what were the necessary changes to get it to work