2021.2/builtin target custom editor

[esmelusina]

backport: #6921

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Purpose of this PR

Fix for https://fogbugz.unity3d.com/f/cases/1380485/

The BuiltIn Target was not correctly honoring a custom editor GUI the way the URP and HDRP targets do. This change properly updates the TargetSetupContext with the name of a custom editor GUI if one is entered on the Target in the graph settings. This works for both the Lit and Unlit SubTargets.

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Testing status

Manual testing according to the repro case in the bug report. Also added an Unlit ShaderGraph and associated material to ensure it works correctly there. Made sure I hit a breakpoint in the projects custom GUI.

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://unity-ci.cds.internal.unity3d.com/project/902/
Search for your PR branch using the search bar at the top, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

Shader Graph
/jobDefinition/.yamato%252Fall-shadergraph.yml%2523PR_ShaderGraph_2021.2
Depending on your PR, you may also want
/jobDefinition/.yamato%252Fall-shadergraph_builtin_foundation.yml%2523PR_ShaderGraph_BuiltIn_Foundation_2021.2
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[jessebarker]

LGTM