Provide a way to attach data to clients

Originally raised while discussing whether we should have client.isFrozen etc.

There's a lot of state data we could attach to clients, but developers also postMessage clients to 'ask' for some app-specific data, like "is the user currently typing a message?".

We could improve both situations by allowing developers to attach clonable data to a client.

Rough design that I haven't put much thought into:

// From a page (or other client):
await clients.setClientData({ foo: 'bar' });

// In a service worker:
const client = (await clients.matchAll())[0];
console.log(client.data?.foo); // 'bar' or undefined.

Only the client itself will be able to set client data.

My rough API proposal includes adding self.clients to windows/workers. We might be able to avoid adding the whole clients API if we want to take baby steps here.

Start by reviewing the proposed clients.setClientData() and client.data examples, along with the question of exposing self.clients to windows and workers. Clarify the API shape, cloning and ownership rules, and the required service-worker behavior with the maintainers. Done means the design is agreed well enough to specify and implement.