Closes #13086: Virtual circuits (#17933)

* WIP

* Add API tests

* Add remaining tests

* Add model docs

* Show virtual circuit connections on interfaces

* Misc cleanup per PR feedback

* Renumber migration

* Support nested terminations for virtual circuit bulk import

docs/models/circuits/virtualcircuit.md

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+# Virtual Circuits
+
+A virtual circuit can connect two or more interfaces atop a set of decoupled physical connections. For example, it's very common to form a virtual connection between two virtual interfaces, each of which is bound to a physical interface on its respective device and physically connected to a [provider network](./providernetwork.md) via an independent [physical circuit](./circuit.md).
+
+## Fields
+
+### Provider Network
+
+The [provider network](./providernetwork.md) across which the virtual circuit is formed.
+
+### Provider Account
+
+The [provider account](./provideraccount.md) with which the virtual circuit is associated (if any).
+
+### Circuit ID
+
+The unique identifier assigned to the virtual circuit by its [provider](./provider.md).
+
+### Status
+
+The operational status of the virtual circuit. By default, the following statuses are available:
+
+| Name           |
+|----------------|
+| Planned        |
+| Provisioning   |
+| Active         |
+| Offline        |
+| Deprovisioning |
+| Decommissioned |
+
+!!! tip "Custom circuit statuses"
+    Additional circuit statuses may be defined by setting `Circuit.status` under the [`FIELD_CHOICES`](../../configuration/data-validation.md#field_choices) configuration parameter.

docs/models/circuits/virtualcircuittermination.md

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+# Virtual Circuit Terminations
+
+This model represents the connection of a virtual [interface](../dcim/interface.md) to a [virtual circuit](./virtualcircuit.md).
+
+## Fields
+
+### Virtual Circuit
+
+The [virtual circuit](./virtualcircuit.md) to which the interface is connected.
+
+### Interface
+
+The [interface](../dcim/interface.md) connected to the virtual circuit.
+
+### Role
+
+The functional role of the termination. This depends on the virtual circuit's topology, which is typically either peer-to-peer or hub-and-spoke (multipoint). Valid choices include:
+
+* Peer
+* Hub
+* Spoke