return user id (UUID) in the X-Okapi-User-Id header

It was also added to the token as user_id.

Guys, just a note that the X-Okapi-User-Id should include the UUID of the user (not the user login name, since we are dropping it's use as a foreign key, see https://folio-org.atlassian.net/browse/MODUSERS-30#icft=MODUSERS-30).

@Kurt Nordstrom Will this also mean that the contents of the token changes?

Just like with the tenant-Id, mod-authtoken should check that if it sees a X-Okapi-User-Id header, then the value should match the one in the token. That way, malicious clients can try to set up a different one, and we will catch them. If it does not see a X-Okapi-User-Id header, it should just extract the value from the token, and return that in X-Okapi-User-Id.