This untangles cross-signing and secret storage setup into separate path that
can be invoked independently. There is no functional change with this patch, but
instead this just separates one giant monster API into two.
Part of https://github.com/vector-im/element-web/issues/13895
This converts all secret storage to use a newer account data API which uses
cached data in stored when available, but also knows how to ask the homeserver
in case it's invoked during early client startup before the initial sync.
As a consequence, it means most secret storage APIs are now async.
Part of https://github.com/vector-im/riot-web/issues/11901
The earlier commit, d3ce0cb82f, has most of the juicy details on this. In addition to d3ce's changes, we also:
* Use `TestClient` in many integration tests due to subtle behaviour changes in imports when switching to ES6. Namely the behaviour where setting the request function is less reliable in the way we did it, but `TestClient` is very reliable.
* We now use the Olm loader more often to avoid having to maintain so much duplicate code. This makes the imports slightly easier to read.
This changes `addKey` for secret storage to consume info about a pre-generated
key, rather than creating the key in middle of the method. This eases UI work
that want to have the public and private keys earlier on in the flow.
It's probably important that the app knows if a secret isn't going
to be stored under one or more of the keys it thought it was going
to be stored under.
Also add a test to assert it.
