Read receipts from accounts who are not room members aren't displayed

### Steps to reproduce

When having bridged rooms (i.e. WhatsApp rooms) where the bridge bot sends `m.read` receipts to messages, these read avatar icons are only shown on Element iOS if the account that sent the read receipt is a member of the room (invited but not accepted is enough).

On Element Web these read markers are shown with initials, but no avatar is loaded.

The desired functionality would be if all read receipts are displayed in the iOS App (no matter if the account sending the read receipt is a member of the room or not). Bonus would be, if the avatar of the account gets shown correctly, rather than just the initial.

#### Reproduce
- Create a DM room with another account
- Create a read event sent from an account that is not a member of the room (i.e. a bot account)
- Element iOS won't display that read receipt

### Outcome

#### What did you expect?
- Being able to see the avatar of all accounts that sent read receipts for messages. (Element web shows at least the initials, not also not the avatar)

#### What happened instead?
- No read receipt is shown at all, also when tapping on the displayed read receipts, the popup of all read receipts has no hint for the missing read receipts at all

### Your phone model

iPhone 13 Pro

### Operating system version

15.2.1

### Application version

Element 1.8.1

### Homeserver

Synapse 1.52.0

### Will you send logs?

No