feat: support icon upload for non-hosted apps (Bolt Framework apps)

[gabrielmouallem]

Description

The apps.hosted.icon endpoint currently rejects non-hosted app IDs, which means traditional apps created via apps.manifest.create (e.g., Bolt apps) can't set their app icon programmatically.

There's already a TODO in internal/pkg/apps/install.go:525 acknowledging this:

// TODO: Currently, cannot update the icon if app is not hosted.

…followed by commented-out code that would handle icon upload for non-hosted apps, identical to how it works for hosted deploys (lines 220-233).

It would be great to extend apps.hosted.icon (or introduce a new endpoint) to accept non-hosted app IDs so all Slack apps can set their icon programmatically. This is also related to slackapi/manifest-schema#42 where Slack staff mentioned a public API for this is in the works.

cc @mwbrooks

Requirements

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

[mwbrooks]

Hey @gabrielmouallem 👋🏻

You've read our wishlist and done your homework. Thanks for taking the time to dig around the Slack API Icon Endpoint and Slack CLI codebase.

You're correct that the icon endpoint only works for hosted (Deno SDK) apps. This is enforced by the Slack API, so we can't solve it directly in the CLI codebase.

However, a new API endpoint is being introduced that will support non-hosted apps (and probably hosted apps). So, soon we'll be able to switch it over so that you can configure the icon from your local app and it'll be respect on App Settings (https://api.slack.com/apps).

Originally, we were hoping to make this happen by the end of March, but it won't make the upcoming release. We'll try to have it available by mid-April. When we open a CLI PR, we'll try to update this issue so that you can try it out while it's under review!