Support pgp encrypted messages

### Your use case

#### What would you like to do?
Decrypt pgp-encrypted events inside of Element:

#### Why would you like to do it?
Another Matrix client, [Quickmedia](https://git.dec05eba.com/QuickMedia/about/) has recently added support for users to send pgp-encrypted events, where users can add each others' published keys.

#### How would you like to achieve it?
Add an option for Element to automatically decrypt these events

### Have you considered any alternatives?

I can copy the text to a file and decrypt it with gpg myself but that's not as convenient as what Quickmedia has.

### Additional context

_No response_

### Are you willing to provide a PR?

No

Start by reviewing how Element clients represent and render events, then compare the pgp-encrypted event behavior described for QuickMedia. Identify the supported encryption and key-handling boundaries before defining an implementation path. Done means Element can automatically decrypt these events using published keys and display the plaintext safely.

Written by the indexing model from the issue text.