SUP-2777 - Allow configurable pipeline slugs

[petetomasik]

Changes slug from a read-only attribute to an optional configurable attribute. If not set, slug is derived from the pipeline name and returned from the API. The slug will be updated when the name of the pipeline changes. If the slug is defined, a second REST API call is made to update the slug for the pipeline. The slug can be set or excluded at any time in the pipeline's lifecycle and the value will be set based on the user-defined input or via the API's response.

Utilizes Terraform private state to track whether slug value in State was set via api or user. This extra value is required to correctly evaluate the transition of removing user-defined slug from an existing resource and re-trigger storing an updated slug from the API.

Addresses: #586