feat: Identify webContents of renderers via custom protocol

[timfish]

The Electron SDK supports two ways for the renderers to send envelopes and scope to the main process.

• Electron IPC - Preferred but requires a preload script which we attempt to inject automatically but this fails when bundling the main process code
• Custom protocol + fetch - No setup required but Electron does not give us a way to determine which renderer the event came from ([Feature Request]: Add sender: WebContents to ProtocolRequest electron/electron#31718)

This has meant custom protocol has not supported all the same context as Electron IPC because we're missing the reference to the webContents.

This is a problem for renderer ANR detection. We should be able to use the Electron debugger API to capture stack traces but once a renderer stops sending us mesages, we need to know which webContents to attach to!

This PR adds a means for us to identify renderers over a custom protocol:

• In protocol mode, window.__SENTRY_RENDERER_ID__ is set to a uuid
• When a renderer starts up and pings the main proces via the custom protocol, we use executeJavaScript to fetch the id and save the mapping between id and webContents
• When the renderer sends events/envelopes/scope and later ANR messages, the id is included in the headers
• In the main process, we can use the id in the headers to determine which webContents the message came from

[timfish]

I want to add an extra test for this before merging