[identity] Update SignedDeviceList::from_str to return tonic::Status

Summary:
Noticed that we're using type Error = serde_json::Error in FromStr, but we map it everywhere to tonic::Status.
Refactored this to map it only in one place and avoid adding another mapping in the next diff

Depends on D13222

Test Plan: cargo build -p identity

Reviewers: varun, will, kamil

Reviewed By: will, kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13223