Schema generator

[jirikuncar]

Add support for schema generation for given metadata from Object Graph Mapper.

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• original discussion on https://github.com/davebshow/goblin/issues/85#issuecomment-337960118
• simple generator https://gist.github.com/davebshow/e788e3c477bea8464864b2ca2be5100f

[davebshow]

As I mentioned in #85 I would like things like this to go in a separate repo goblin-janus. Would you like to host? If not, I can.

[jirikuncar]

As much as I like to approach with separate repo, I would like to see the schema generator being an integral part of the Goblin library.

If you decide for a separate repo, I think it's a time to create an organisation for goblin/aiogremlin related repos would be nice.

[fridex]

We use "goblinoid" successfully for some time - see https://github.com/thoth-station/goblinoid and https://pypi.org/project/goblinoid/