Adding support for Additional Hooks Paths

[CerealBoy]

Description

We would like to provide the ability to place hooks in multiple locations, meaning there can be multiple hooks of the same type, and they can be provided under different circumstances. These additional hooks are comma separated list, and ordered as they are provided in terms of how multiple hooks are executed. Those in hooks-path are always first, and then sequentially through additional-hooks-paths.

Changes

• Adding BUILDKITE_ADDITIONAL_HOOKS_PATHS configuration option
• Find and run any hooks found after hooks-path

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)