Add grease tokens

[Auth Scheme specification](https://www.ietf.org/archive/id/draft-ietf-privacypass-auth-scheme-15.html#section-4.1) presents a Greasing section in 4.1.

The library should provide a Greasing token type, along with associated test vectors. This would allow for a healthier ecosystem, and an easier implementation of greasing by origins.

Start with the linked Auth Scheme specification, especially the Greasing section in 4.1, then locate the library's existing token types and test-vector tests. Done means the library exposes a Greasing token type and includes associated test vectors; the issue names no repository files or test entry points.

Written by the indexing model from the issue text.