Closes https://github.com/fastapilabs/cloud/issues/2423, https://github.com/fastapilabs/cloud/issues/2424, https://github.com/fastapilabs/cloud/issues/2425

Needs https://github.com/fastapilabs/cloud/pull/2712

This adds new commands in the command palette to sign in and out using a device flow, as well as commands to link and unlink an app. This also works in both local and in browser contexts (e.g. vscode.dev/github.dev).

The new status bar in the bottom left has a couple states:

• Not signed in → "Sign into FastAPI Cloud"
• Signed in, no app linked (i.e. no .fastapicloud/cloud.json) → "Set up FastAPI Cloud"
• Signed in, config exists but app returns 404 → "⚠ Set up FastAPI Cloud". When users click this, they will get a one-time warning toast that tells them that something has gone wrong and suggest they unlink and relink their application
• Signed in, app linked → shows app slug

In "Set up FastAPI Cloud" state, clicking the status bar shows a quick pick:

• "Link Existing App" — connects to an existing app
• "Create New App" — creates a new app and links it

In the "⚠ FastAPI Cloud" state, clicking the status bar shows a warning toast prompting the user to unlink and relink.

When an app is linked, clicking the status bar shows Open App, Dashboard, and a More... menu with Unlink and Sign Out.

Users will also now see their auth status in the accounts menu as well.

More details:

• In local contexts, we store the sign in token in the same place that the CLI does, so that users never have to double auth. In vscode.dev, we store in SecretStorage, since the file doesn't exist.
• There is also a setting fastapi.cloud.enabled which controls whether the cloud features are enabled (they are enabled by default but I figured that there may be users who don't want them at all and I'd like to give them a way to declutter if they aren't interested).
• Also instrumented new events for tracking sign in, sign out, app view, dashboard view, link and unlink.
• I will create a new VSIX for y'all to test once I get the deploy command in there, so it's actually usable 😅!