Scan attachments for virus

[bmarty]

RiotX should optionally be able to communicate with an antivirus server to scan all attachments of a room before displaying them or let the user download or open it.

This will be similar to what have been done on Tchap (API/UX-UI):

• Files are uploaded to the media repository as normal
• When downloading a file, the client send its decryption key to the server, itself encrypted using a public key provided by the content scanner
• The content scanner uses the decryption key to decrypt the file and scan it against current virus definitions
• If the media is virus free, the encrypted file is returned to the client and the decrypted media securely removed from the server
• The scan result is cached by the content scanner so that if somebody else asks for the file again, it can be provided without being decrypted
• The cached result is persisted until virus definitions are updated or the server is rebooted

Ref:

• https://github.com/matrix-org/matrix-doc/issues/1453
• https://github.com/matrix-org/matrix-content-scanner

on client side:

• https://github.com/dinsic-pim/tchap-android/tree/develop/vector/src/main/java/fr/gouv/tchap/media
• https://github.com/dinsic-pim/tchap-android/tree/develop/vector/src/main/java/fr/gouv/tchap/model
• Finally the restClient is already in old matrix-android-sdk (see https://github.com/matrix-org/matrix-android-sdk/blob/develop/matrix-sdk/src/main/java/org/matrix/androidsdk/rest/client/MediaScanRestClient.java)