Create API Key expiresAt field

[AllenZheng-05]

The POST /admin/apikeys endpoint creates an api key. the expiresAt field is optional, but if not included it defaults to being automatically expired. Could change to:

1. make the field required and throw an error if not included
2. default to expiring a year from now if not included
3. make the key never expire if not included (probably not this one)