feat(invoke): add invoke handlers for blocks and task

[douglas-reid]

Hot take: Getting raw dicts back from invoke is a usability concern. Since we are pushing AgentService with a builtin prompt(): -> List[Block], we should have an easy way to actually get those blocks directly.

Proposed usage:

answer_blocks = agent.blocks_from_invoke("prompt", prompt="What did I eat for dinner?", context_id="doug-dinner-discussion')

for block in answer_blocks:
   print(block.text)

[eob]

The generalized version of this is something I really wish we had... to get some way to re-build the objects on the other side of the HTTP wire.

[dkolas]

LGTM

[douglas-reid]

@dkolas thanks. updated with tests. ptal.