[ShaderGraph][2022.1] Detect duplicate subgraph input/output slots

[jessebarker]

Purpose of this PR

This PR fixes https://fogbugz.unity3d.com/f/cases/1366200/

The recent updates to fix https://fogbugz.unity3d.com/f/cases/1337137/ do not handle the possibility that a subgraph might have duplicate input or output slots. This largely never happens as the actual ShaderGraph UI (graph editor) does not permit it. However, it is possible that some custom user assets may fall into this category. We should detect this situation, warn the user that they have an unsupported subgraph, and tell them how to update their subgraphs so that they work properly.

Here is an example of the console output the user would see (based on the repro project):

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Testing status

Manual testing according to the repro steps in the bug.

N.b., On this Yamato run, the bokken farm was having certificate issues on the OSX agent (also present on master at branch-off for this PR)

Comments to reviewers

Notes for the reviewers you have assigned.

[github-actions]

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