I have a service account that wants to perform an operation as a user using the "As-User" header (or do some other operation regarding a user). However, the code that is going to be making these calls only has the user's username (which is also their email address - I'm not sure if this is always true, but it appears to be).
How can I resolve a username to a user ID? There doesn't seem to be a REST API endpoint to search for users by username/email.