Enable measures to return rich outputs (plots, HTML tables)

[skaltman]

Addresses #56.

I mostly wanted to see what the TLG agent felt like with plots and gt tables supported as measure outputs, so I started working on this.

Totally fine if we want to go in a different direction for this.

Idea for more general solution from sol:

Future work: Consider generalizing rich_table() into a rich_output() abstraction that
separates user-facing HTML, model-facing semantic content, and a calculation-ready
run_r value. rich_table() could remain a table-specific wrapper, while the same
foundation could support maps, HTML widgets, and other interactive visualizations
without applying table-specific rendering or model instructions.

[github-actions]

Preview deployed to Connect (dogfood.team.pct.posit.it): https://dogfood.team.pct.posit.it/connect/#/apps/d7a36cae-8f27-448b-a478-61b81fbe3942/draft/361860

Deployed from commit 09f866a.

[github-actions]

Preview deployed to Connect (connect.staging.pct.posit.it): https://connect.staging.pct.posit.it/connect/#/apps/ad662e1b-5048-4acc-9ad7-f9478c92274e/draft/1809

Deployed from commit 09f866a.