Multiple account

### Your use case

#### What would you like to do?

Login to multiple accounts from the same Element app.

#### Why would you like to do it?

To access multiple accounts simultaneously.

#### How would you like to achieve it?

Ideally I would be able to switch between seeing a mix of rooms from all accounts and restricting the view to one account.

### Have you considered any alternatives?

I've been using multiple different clients on my phone for years, but different clients have different featuresets.

### Additional context

Because of the rewrite this would be the perfect time to at least make sure nothing in the infrastructure assumes a single account. If that assumption is held from the start then it will be harder to allow multiple accounts later.

https://github.com/vector-im/element-meta/issues/1831

### Are you willing to provide a PR?

No