Consider alternatives for reading OpenAPI and metadata dependencies

[zengin]

Describe the bug
Currently DevX API reads metadata and OpenAPI documents directly from msgraph-metadata GitHub repo, which makes it susceptible to issues like: microsoftgraph/msgraph-metadata#172

Expected behavior
Production sources should be guarded with additional checks than a mere write access to a GitHub repo, especially when an automated pipeline has direct write access (as in the case of generation process).

[ddyett]

agreed with this, there isn't sufficient access control.

[darrelmiller]

If we change the production of clean metadata to be based on the schemas folder, we will have the schemas update PR as a gate. Currently we are pulling from $metadata without a gate. Moving to the schemas will address a range of different issues.