Proposal: change webid claim to solid

This is a proposal to change the webid claim to solid in access tokens and ID tokens.

The background for this is severalfold:

1. The current webid claim is very WebID specific, and WebIDs are (according to the draft WebID specification) limited to HTTPS URLs. If other types of identifiers are to be supported (e.g. DIDs, VCs), placing those in the webid claim is questionable. A solid claim would therefore be more flexible and, arguably, forward looking.
2. The names used by Solid-OIDC have generally been moving toward "Solid" and away from "WebID". The specification name is Solid-OIDC (it was formerly WebID-OIDC). The audience claim for access tokens uses a value of solid to indicate that the token should be used with the Solid ecosystem.
3. WebIDs will continue to be supported with a solid claim and will likely continue to be the main identifier format for agents in the near term
4. There is a discussion to use a scope value with Solid-OIDC, and there is an indication that this scope could be solid. If the name of that scope is, in fact, solid, then using a solid claim in the resulting tokens would make for a simple, consistent naming structure.

If the name of this claim is changed to solid, we should constrain the value(s) to be IRIs.

This change would place no new requirements on Solid components to support DIDs, but it does make support of DIDs more possible for the future.

This change would require adjustments on client apps (RP), Pod servers (RS) and identity providers (OP).

Start by reading the Solid-OIDC specification sections defining access-token and ID-token claims, then review the discussion in issue 29 about the proposed scope value. Done means reaching agreement on the claim name and IRI constraint and updating the specification to account for the affected RP, RS, and OP components.