Support custom target authoring SDK for third-party deployment targets

[astrobot-houston]

Originally proposed by @NathanFlurry in #515. Their description is reproduced verbatim below.

All community-submitted pull requests are automatically converted to issues (bugs) & discussions (feature requests, enhancements) where they can be triaged and prioritized. Once prioritized, a PR implementation is created automatically.

• Add the target authoring API and load custom target packages through Flue configuration.
• Cover custom target builds with an end-to-end CLI test.
• Document the Rivet target and Rivet/Vercel deployment flows.

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Original implementation from #515 by @NathanFlurry