Implement external_select block kit for Slack

[Franz1241]

external_select as per Slack Docs

Summary

• Adds <ExternalSelect> modal element backed by Slack's external_select block, so modals can populate dropdowns from a remote/large data source instead of being limited to static <Select> options
• Adds onOptionsLoad(actionId, handler) for registering loaders. Handlers receive {actionId, query, user, adapter, raw} and return normalized {label, value}[]. Specific action IDs run before catch-all handlers; per-handler errors are isolated
• Slack adapter now handles block_suggestion interactive payloads, calls chat.processOptionsLoad, and converts results into Slack option objects (capped at 100, per Slack's limit).
• Loader execution is capped at 2.5s to stay inside Slack's 3s response budget — on timeout the adapter still returns 200 with an empty options array and logs a warning, rather than failing the request or leaving Slack hanging.
• Documentation

Test plan

-pnpm validate runs 16/16

Closes #398

[vercel]

@Franz1241 is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[Franz1241]

Hey @dancer hope its okay to tag you. I noticed youve worked on some Slack-related PRs recently and thought you might be the right person to review this
Id appreciate it if you could take a look when you have time
This ft would help unblock my work

[dancer]

ping me anytime and will take a look later today