rofl-appd: allow 0x prefix in "to" field in /sign-submit

[lukaw3d]

could rofl-appd.sock /sign-submit be made a little more permissive so that "to": "0x1234845aaB7b6CD88c7fAd9E9E1cf07638805b20" was allowed, not just "to": "1234845aaB7b6CD88c7fAd9E9E1cf07638805b20"

somewhat easy mistake to make in changing the env variable
https://github.com/oasisprotocol/demo-rofl/blob/c2e085fb5e77b19bd1a28cf6e44242343fe48c7d/compose.yaml#L9
https://github.com/oasisprotocol/demo-rofl/blob/main/docker/app.sh#L18C64-L18C93
(if we want to keep it strict, I suggest updating all our examples to ${CONTRACT_ADDRESS/0x/} and adding a new error message for this case. current error if not silenced:

 "msg":"\"reason\": \"Unprocessable Entity\"
 "msg":"\"description\": \"The request was well-formed but was unable to be followed due to semantic errors.\""

)