Create a new JSON-LD Context for VCs containing RSS proofs

[thobson88]

To fully support RSS proofs in VCs, the attributes of the RSS proof must be found in the JSON-LD context that is referred to inside the VC.

No such context currently exists (because our implementation of RSS signatures for VC selective disclosure is novel).

In the absence of such a context, our temporary workaround has been to change the expansion policy (that's used e.g. when expanding a VC in the process of signing a VP), setting it to Relaxed rather than Strict. (This was done in this commit on our fork of the SSI repo). Without this workaround, the process of generating a VP from a VC fails.

However this change in the expansion policy causes two test failures in the SSI codebase (albeit in crates for DID methods unrelated to Trustchain).

The steps needed to properly fix this issue are:

• create a suitable JSON-LD context
• load that context when handling VCs with RSS proofs (see the related issue #128)
• undo the temporary workaround by setting the JSON-LD expansion policy in the SSI library back to Strict.