feat: implement support private docker repo support with platform clusters

[mikeshultz]

Overview

Implement support for managing Docker credentials on platform clusters.

Specification

Relevant endpoints (see openapi docs for details):

• GET /credentials
• POST /credentials
• DELETE /credentials/:credentials_id

[linear[bot]]

SBK-517 feat: implement support private docker repo support with platform clusters

[fubuloubu]

Should it also have PUT /credentials/:credentials_id?

[mikeshultz]

Should it also have PUT /credentials/:credentials_id?

That verb wasn't implemented in the backend. Though, we probably do want it.