[GRAPHCDN] add initial spec

[cstrnt]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Feature

What is the current behavior? (You can also link to an open issue here)
No spec in place

What is the new behavior (if this is a feature change)?
There is a spec

Additional info:

[withfig-bot]

Overview

dev/graphcdn.ts:

Info:

[withfig-bot]

Hello @cstrnt,
thank you very much for creating a Pull Request!
Here is a small checklist to get this PR merged as quickly as possible:

• Do all subcommands / options which take arguments have the arg property (arg: {})?
• Are all options modular? E.g. a -u -x instead of -aux
• Have all other checks passed?

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[QuiiBz]

LGTM, GraphCDN is so cool!