Add Playground Example folder and Observable source asset test

[schrockn]

Summary & Motivation

This PR serves two purposes:

1. To introduce an example I could load into dagster-webserver to test Wrap observable source assets in AssetsDefinition #16620
2. To add a generalized dumping ground for simple, standalone examples to keep under test. Often you want to have something you can easily load in the UI, but don't want to make any entire new module for. This provides a place where you can dump that sort of thing.

How I Tested These Changes

[schrockn]

• Add Playground Example folder and Observable source asset test #17063 👈
• Wrap observable source assets in AssetsDefinition #16620 : 1 other dependent PR (#16621 )
• Extract _get_op_def_compute_fn into wrap_source_asset_observe_fn_in_op_compute_fn #16618 : 1 other dependent PR (#16619 )
• Source Asset -> Unexecutable Asset Adapter #16617
• master

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[alangenfeld]

whats the difference between this new examples playground and python_modules/dagster-test/dagster_test/toys/ ?

[schrockn]

I think nothing. I'll move it.

[alangenfeld]

ill move it