create incident catalog entry attribute

[crockeo]

Creates a new resource type, incident_catalog_entry_attribute which can be used to assign a single attribute to an Incident.io catalog entry.

Example usage:

resource "incident_catalog_entry_attribute" "attribute" {
    catalog_entry_id = "FAKEID1"
    attribute_id     = "FAKEID2"
    value            = "some value"
}

This form allows you to manage a catalog entry and its attributes separately, which means:

• You can more easily conditionally attach attributes to an entry (using count = <condition> : 1 ? 0, rather than conditionally populating elements in array).
• You can resolve cycles between catalogs, where for example, catalog1 and catalog2 have attributes which reference each other. Currently, that produces a cycle at TF plan time. In this formulation you can provision catalog1, catalog2, and their attributes such that they don't have a cycle.

Note that this architecture is used elsewhere in the TF ecosyste, for example AWS once suggested developer use ingress and egress on the aws_security_group resource, but now they suggest that developers create separate resources, much in the same way.

Note that this creates a sharp edge, where one could accidentally try to manage a catalog entry attribute through both incident_catalog_entry and incident_catalog_entry_attribute. I don't know a great solution to this w/o a backwards incompatible change.