add oneroster 1.2 specification

[brandonopened]

Add the oneroster spec. here's the overall document; - i think the openapi file below is more useful:

https://www.imsglobal.org/spec/oneroster/v1p2

https://purl.imsglobal.org/spec/or/v1p2/schema/openapi/onerosterv1p2rostersservice_openapi2_v1p0.json