Support JSON Schema for VCs

[clehner]

A credential's credentialSchema may specify a URI for a JSON Schema file for validating the credential. (https://w3c.github.io/vc-data-model/#proofs-signatures)

A Rust library for JSON Schema is jsonschema.

jsonschema uses HTTP client library reqwest which pulls in a lot of dependencies (#18). Also, the requests are blocking and responses are not cached (Stranger6667/jsonschema#75). But it looks like the resolver is not used internally to validation but only if the caller calls it. So this might not be a problem us, especially if reqwest is made optional (Stranger6667/jsonschema#137), as we can do our own HTTP resolution (edit: maybe just using reqwest as well), e.g. alongside resolution for JSON-LD Context documents.

Another library is schemafy (mentioned in #21 (comment)) which generates types from schemas at compile time. But it doesn't have full support for the latest JSON Schema drafts. Its use may be limited here since I think we want to be able to validate credentials against arbitrary schemas at runtime.

[wyc]

Adding context here from the CCG ecosystem: w3c-ccg/community#132

Check out the Workday spec draft, maybe a later version here: https://w3c-ccg.github.io/vc-json-schemas/

Note that CCG-hosted specs are not official standards or recommendations, but are a good focal point to consider due to industry and implementer participation, with the possibility of being adopted as a standard at places like W3C and IETF.