Flesh out The Workload Identity Token section

[bc-pi]

Yaron Sheffer
[8:31 AM May 30 in slack]
To summarize the near term plan:
Joe: security considerations and interaction with TLS
Brian: ID Token and DPoP-inspired
Yaron: Message Signatures

This PR is the "ID Token" part.

A preview editors' copy of this PR can be seen at http://www.sheffer.org/wimse-s2s/bc-workload-id-token/draft-sheffer-wimse-s2s-protocol.html

[yaronf]

Also, as discussed, please add a header definition in the following section.

[yaronf]

@bc-pi Can you also include the private key of the service (the one that corresponds to the public key included in the WIT) so we can extend the example, e.g. for message sigs.

[bc-pi]

@bc-pi Can you also include the private key of the service (the one that corresponds to the public key included in the WIT) so we can extend the example, e.g. for message sigs.

Yeah, I'll add it somewhere with those examples. In the meantime though:

{
 "kty":"OKP",
 "crv":"Ed25519",
 "x":"_amRC3YrYbHhH1RtYrL8cSmTDMhYtOUTG78cGTR5ezk",
 "d":"G4lGAYFtFq5rwyjlgSIRznIoCF7MtKDHByyUUZCqLiA"
}