Function from_vcap_services 

pub fn from_vcap_services(
    json: &str,
) -> Result<Option<ServiceBinding>, ServiceBindingError>

Parses VCAP_SERVICES JSON and extracts an ProofVisor binding.

A service entry matches if any of the following are true:

• The top-level key equals "proofvisor" (managed service).
• The entry’s "label" equals "proofvisor".
• The entry’s "tags" array contains "proofvisor".
• The entry’s "name" equals "proofvisor".

User-provided services (created with cf cups) appear under the "user-provided" top-level key, so matching by label, tag, or name is needed to find them.