feat(codebuild): Add an API to list all AWS CodeBuild accounts

[Kaixiang-AWS]

This API is used by deck to list all CodeBuild accounts configured in igor. As CodeBuild is different from other build services(Jenkins, Travis), it can't reuse the existing list accounts API.

Related Issue: spinnaker/spinnaker#5292

[clareliguori]

Can you describe this more?

As CodeBuild is different from other build services(Jenkins, Travis), it can't reuse the existing list accounts API.

[Kaixiang-AWS]

Igor is originally designed for Jenkins and assuming all other CI services are like Jenkins. Therefore it expects all CI services to fit into the framework it designs. For example, if I want to put codebuild into this framework, I have to implement a getBuild method which use build number as input. However, codebuild cannot retrieve a build by its build number. There are other places that codebuild cannot fit into this framework. Therefore, I implemented a similar, but much simpler solution for codebuild. This is exactly how GCB does because GCB also couldn't fit into that framework. There is someone working on refactoring igor to better abstract core concepts. spinnaker/igor#594 For now the only option is implementing codebuild's own logic outside of this framework.

[clareliguori]

Approved, but merge is dependent on spinnaker/igor#595 getting merged in