Add a way to share your username via QR code

### Your use case

#### What would you like to do?
I'd like to easily be able to exchange contact information in person

#### Why would you like to do it?
So that I can message them

#### How would you like to achieve it?
Provide a screen to share your info or message someone via QR code, similar to how you can with the old Element Android app.

### Have you considered any alternatives?

the alternative is to say/spell out your account name and homeserver to someone so they can add you that way. In my case, since I was already connected to both folks, I sent the username to one of them, and then they started a new chat with the 3rd person.

### Additional context

I would suggest an improvement to the flow on the old Element app. Currently you can scan someone's QR code two different ways (from the All Chats screen):
1. Hamburger menu -> Add by QR code -> Scan a QR Code
2. Green "write" button at the bottom -> Start Chat -> QR Code

However, if you want to share your own QR code, only (1) above works, which is a bit confusing. (2) is more discoverable, but then you don't see a way to share, but know one has to exist since you can scan it. Then I go to the Profile screen (clicking on my avatar), as that seems likely to have a way to share my information as it implies a kind of profile view of your account. But no, it's in a 3rd menu on the side. Element X doesn't have this third menu, so maybe this isn't useful context, but adding it here in case it is.

Given how Element X works, adding a new entry to the "Start chat" button with the QR code flow that includes both scanning and sharing seems like it'd be easy to discover.

### Are you willing to provide a PR?

No