Use new token format when generating new id tokens

[jsdt]

Description of Changes

Send tokens with iss and sub claims, and an identity computed from those. This is compatible with the way we are currently parsing tokens, so it doesn't change much code.

API and ABI breaking changes

If this is an API or ABI breaking change, please apply the
corresponding GitHub label.

Expected complexity level and risk

2. This is a low risk change that just depends on clients allowing tokens with more fields. When the server parses incoming tokens, we don't require these new fields yet.

Testing

No testing beyond CI checks here and on the corresponding private PR.

[cloutiertyler]

This LGTM