feat: add demo command

[paegun]

Description

Add a demo command to the spicedb cli.

When spicedb is executed with the demo command and a sample dataset name, a test server is launched and the same dataset is loaded.

Sample datasets harvested from https://github.com/authzed/examples/blob/main/schemas .

Testing

Manually ran each demo sample dataset and used zed to connect and permission check for:
[X] subject granted permission to resource
[X] subject is not granted permission to resource
[X] subject does not exist
[X] resource does not exist
[X] permission does not exist

For example:

spicedb demo --dataset github

zed permission check repository:authzed_go admin user:jimmy

References

#24

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[paegun]

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