See #528, some folks are having issues because their output types are
not strict-compatible.
My approach was:
1. Create `AgentOutputSchemaBase`, which represents the base methods for
an output type - the json schema + validation
2. Make the existing `AgentOutputSchema` subclass
`AgentOutputSchemaBase`
3. Allow users to pass a `AgentOutputSchemaBase` to
`Agent(output_type=...)`
Allows passing in the previous_response_id to reduce sending the same
data again and again.
Test plan:
Examples. Adding tests in next PR shortly.
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[//]: # (BEGIN SAPLING FOOTER)
* __->__ #509
* #508
