Add expires_in to token endpoint response

As reported in: https://github.com/cds-hooks/sandbox/issues/65

When launching the CDS Hooks Sandbox via the HSPC Sandbox, the provided OAuth 2 access token lacks the expires_in value. This prevents the CDS Hooks Sandbox from being able to specify it in the fhirAuthorization field of the CDS Service request. Per the CDS Hooks spec, the expires_in field is required.

Here is an example request to the token endpoint that illustrates the issue: