Custom Relationship REST API does not honor filter restrictions

[nniehoff]

Environment

• Python version: 3.7
• Nautobot version: 1.2.10 & 1.2.11b1

When specifying a filter in a custom relationship the UI limits the choices on that side of the relationship to match the filter. However, it is possible to use the API to create a relationship-association which violates this filter.

Steps to Reproduce

1. Create a 2 sites (object types really don't matter) site1 and site2
2. Create a VM Cluster cluster1
3. Create a relationship 1-1 relating a virtualization | cluster to a dcim | site using a filter such as:

{
    "name": "site1"
}

4. Navigate to the cluster in the UI and attempt to relate the cluster to site2, in the UI you should see the choices are limited to site1.
5. Use the API to POST to /extras/relationship-associations/ something like (replacing the ids with the source_id of cluster1 and the destination id of site2):

{
  "relationship": "0257b6fb-d8a8-42af-9b60-8e00cc06cce6",
  "source_type": "virtualization.cluster",
  "source_id": "3deaaaa3-a859-4295-a586-3bf71db19f00",
  "destination_type": "dcim.site",
  "destination_id": "cf228b2c-ef24-4169-9070-626e9337b992"
}

Expected Behavior

The UI and the API should be consistent. The API should fail to create the relationship-association as this does not match the given filters.

Observed Behavior

The API creates the relationship with site2 which is invalid in the UI.