fix: cached plugin events respect the specified client id

[mfasold]

Background: While experimenting with custom events, I found that some of the events that targeted a specific plugin instance were instead sent to all plugin instances. It turned out that events are cached until a plugin is loaded. But while all un-cached events are sent to the client that is specified in PluginInstruction::Update, this is not the case for cached events.

I also found that this applies not only to my experimental events. Also, some standard events (e.g. TabUpdate) with specified client ids are among the cached events. So I suspect this fix might not only benefit future events, but also makes current cached events more specific.

Description of the changes: WasmBridge::cached_events_for_pending_plugins now includes the Option<ClientId> specified in PluginInstruction::Update. The function apply_cached_events will respect these client ids, if provided.

I consider this a small fix, so no separate issue has been opened.

All normal tests run fine. Unfortunately I was not able to get the E2E tests running on my Mac, so I could only do some manual tests.