Integrate proxy v2 into SDK

The SDK will now unconditionally use proxy v2: in development mode,
it'll shell out directly, while production mode will invoke it over
qrexec. The dev `./run.sh` script will compile the proxy so it's ready
for use before the client starts up.

New typed dataclasses represent the two types of responses that can be
returned. No user-facting changes are happening at this stage, but this
will enable future client features.

The union return of send_json_request() means that we need instanceof
assertions to make mypy happy.

One minor logic bug in API.get_submission() was fixed in the case that
no request is made (an undefined exception would've been raised previously).

client/run.sh

@@ -39,7 +39,10 @@ echo ""
 
 cleanup
 
-gpg --allow-secret-key-import --import tests/files/securedrop.gpg.asc &
+gpg --allow-secret-key-import --import tests/files/securedrop.gpg.asc
+
+echo "Building proxy..."
+(cd ../proxy && cargo build)
 
 # create the database and config for local testing
 poetry run python create_dev_data.py "$SDC_HOME" &