Validate signature for testbed status data

[yoursunny]

Currently, the testbed status data packet is retrieved without verifier.
The application should validate the signature before accepting the data.

Trust Schema Validation

The application may define a Light VerSec trust schema and use the LVS validator.
See examples in NDNts-NLSR-status page:

1. write a schema file
2. use python-ndn to compile the schema into binary
3. use printESM to convert the binary schema into NDNts native format
4. import the schema and pair with trust anchors
5. pass the validator to both metadata retrieval and segmented object retrieval

Fallback

If the data cannot be retrieved due to:

• unable to connect to NDN network
• the only file server is down (see Replicate routers.json to multiple routers #10)
• unable to validate signature

The web application should retrieve https://testbed-status.named-data.net/testbed-nodes.json and https://{host}/file-server/status.json via Fetch API, which contains the same data.