Specification Proposal: Support for nodegraph inheritance

# Nodegraph Inheritance Proposal

This proposal is to allow for one nodegraph to inherit from another nodegraph.
This is for the same purpose of any existing support for inheritance including per node, and per nodedef.

There possible interpretations are as follows, given a nodegraph B which inherits from nodegraph A:

1. B only inherits the interface of A but the "internals" (nodes in the nodegraph) do not necessary need to match A.
2. B inherits all the elements( nodes, inputs, outputs and interfaces) of A.
3. B inherits all internal elements (nodes, inputs, outputs) but not interfaces of A.

1, and 2 are "additive" model in which the interface cannot change.
3. Allows for arbitrary interface changes.

# Properties:
1. There are interfaces to query only those elements belonging to B.
2. There are interfaces to query all elements in A and B.
3. All existing nodegraph associations can still be supported including:
- usage as compound nodegraphs
- usage as functional nodegraphs. Supporting nodedef and implementations to both use inheritance.
- association via implementation elements (part of 1.39 specification addition).

# Examples

`USDUvTexture` definition. (Note that all outputs must be defined due to an issue which exists at time of writing with signature resolution).
```xml

```
Implementation with inheritance with additional output.
```xml

```
Current implementation:
```xml

```

No files, tests, or entry points are identified. The proposal presents three inheritance models and requests query interfaces plus compatibility with existing nodegraph associations; the implementation's completion criteria would need to be defined before work can begin.

Written by the indexing model from the issue text.