[Feat] [Parent Tracker Issue] Implement the spec changes needed to align with v0.4.0

The v0.4.0 milestone tracker issue is here: https://github.com/a2aproject/A2A/milestone/2

I have created sub-issues for this based on changes that have already been merged to the https://github.com/a2aproject/a2a and based on the issues currently raised for the v0.4.0 milestone (as of Nov. 18th).

**NOTE: We need to continue to keep an eye on the upstream issues targeting 0.4.0 to continue to incorporate more things as they get merged and as new issues are added to the milestone:**

https://github.com/a2aproject/A2A/milestone/2