Add Structured Output

[collindutter]

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Describe your changes

Added

• Structured Output support for all Prompt Drivers.
• PromptTask.output_schema for setting an output schema to be used with Structured Output.
• Agent.output_schema for setting an output schema to be used on the Agent's Prompt Task.
• BasePromptDriver.structured_output_strategy for changing the Structured Output strategy between native, tool, and rule.

Issue ticket number and link

Closes #1468

[codecov]

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[collindutter]

@dylanholmes @vachillo still working through docs and some final cleanup, but the PR is certainly developed/large enough for first pass of reviews.

[vachillo]

offline discussion: support a json schema for output_schem, rather than a schema.Schema object

[dylanholmes]

Optional, but I think avoiding nesting is cleaner:

output_schema = prompt_stack.output_schema

if output_schema is None:
    return
    
...