consider merging sec-private-state-token and sec-private-state-token-crypto-version

[colinbendell]

If PST continues to use headers (see #251), then the sec-private-state-token-crypto-version header should be converted into a structured header and should be merged with sec-private-state-token. Ideally something like:

• Use byte-sequence type to represent the tokens (in IssueRequest, IssueResponse and RedeemRequest)
• Add version (and type?) parameters to identify the PST version of the spec.

For example:

sec-private-state-token: :AAEEVFIqN9o3HN==:; v=1; crypto=VOPRF

[dvorak42]

Yeah, I think shifting it so that if we use headers we move all the operations/headers into one header with a structured header would make more sense. Adding vStandard label for that change.