Track endpoints that use types to report more useful errors

[jgallagher]

We were recently making some changes to a type in omicron-common and got this error when trying to generate the sled-agent API:

     Failure sled-agent (versioned v17.0.0 (blessed)): Oxide Sled Agent API
           error: OpenAPI document generated from the current code is not compatible with the blessed document (from upstream)
                  - at .components.schemas.UplinkAddressConfig -> .components.schemas.UplinkAddressConfig2: change: object required properties changed
                    --- a/blessed
                    +++ b/generated
                    @@ -1,21 +1,24 @@
                     {
                    -  "UplinkAddressConfig": {
                    +  "UplinkAddressConfig2": {
                         "properties": {
                           "address": {
                    -        "$ref": "#/components/schemas/IpNet"
                    +        "allOf": [
                    +          {
                    +            "$ref": "#/components/schemas/IpNet"
                    +          }
                    +        ],
                    +        "description": "The address to be used on the uplink. Set to `None` for an Ipv6 Link Local address.",
                    +        "nullable": true
                           },
                           "vlan_id": {
                             "default": null,
                             "description": "The VLAN id (if any) associated with this address.",
                             "format": "uint16",
                             "minimum": 0,
                             "nullable": true,
                             "type": "integer"
                           }
                         },
                    -    "required": [
                    -      "address"
                    -    ],
                         "type": "object"
                       }
                     }

The contents of the complaint make sense - we were changing UplinkAddressConfig::address from required to optional - but it's not obvious what we need to do to fix the problem. The thing we need to do here is add a new version of any endpoints that use this type (or types that contain this type). I manually grepped for usages of UplinkAddressConfig in the blessed OpenAPI spec to find the type(s) that it contained, then the type(s) that contained those types, and so on, until I found the endpoint we had missed, but it would be great if drift could do that and tell us the relevant endpoint(s) in these errors.

[ahl]

I would have expected the operation to appear in the path; @sunshowers is it possible that the openapi manager isn't emitting it?

[sunshowers]

I started working on this but then ran into the fact that we only track one endpoint for each component (see #19 (comment)). In that PR I also refactored things to generate one change instance per component or endpoint, while collecting all the paths from which that is accessible -- this makes the API manager display code very straightforward.