feat: make adk work with a2a-go/v2

[yarolegovich]

• Copied the code to ./agent/remoteagent and ./server/adka2a to v2 directories.
• Migrated the code in v2 directories to use a2a-go v2 types.
• Changed the code in the original directories to delegate to the new code and serve as a type translation layer.
• Deprecated original packages with a reference to v2 directories.

What this gives is that ADK can be used with a2a-go/v2 which implements A2A protocol v1 (as well as v0).