add vendored google-auth for el7

[hopkiw]

• add dependencies for packages that are available
• copy google-auth 1.21.2 and 1.30.1 to vendor/ directory
• update el7 spec to include 1.21.2
• rewrite absolute imports during package build

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hopkiw, illfelder

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/GoogleCloudPlatform/artifact-registry-yum-plugin/blob/main/OWNERS)~~ [hopkiw,illfelder] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment