Add support for OAuth 2 authentication

[fbiville]

Currently, the only supported authentication types for secrets are Basic and SSH Auth.
However, some registries such as Harbor require OAuth2 (as documented here for Harbor).

[jldec]

Perhaps also of use, this post by Zhimin Wen offers a description of how to configure Knative builds to run with GitLab and IBM's on-prem private container registry.

[jldec]

That 👆 came from knative/serving#1996 which includes a broad discussion of the issues related to using Knative with private / on-prem / localhost registries.