feat: support icon upload for non-hosted (Bolt/manifest-created) 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.