feat: use privacy-preserving webxdc addresses

[r10s]

this PR adds the address to be used by the UI for window.webxdc.selfAddr to webxdc-info. UIs need to be changed accordingly and must not use configured_addr any longer.

the address is created by sha256(private-key + rfc724_mid) , which results in different addresses for each webxdc, without the option to find out the real address of the user.

this also returns the same address for a multi-device-setup - sending totally random self address around might be an alternative, however would require connectivity (both devices may be offline on first start).

for existing app, after the change, there will be a new user, resulting eg. in a new highscore, otherwise, things should be mostly fine. this assumption is also important as we might change the thing another time when it comes to multi-transport.

ftr, addresses look like 0f187e3f420748b03e3da76543e9a84ecff822687ce7e94f250c04c7c50398bc now

when this is merged, we need to adapt #6230 and file issues for all UI to use info.selfAddr

closes #6216

[link2xt]

Here is an answer to a similar question regarding security of hashing a secret key:
https://crypto.stackexchange.com/questions/61915/can-i-hash-a-secret-key-and-used-the-hash-as-key-id

[r10s]

@link2xt i changed generation to use fingerprint as discussed