Add context method to get observable asset key

[benpankow]

Summary

Addresses #18896. Adds a new property observable_asset_key on the OpExecutionContext which returns the asset key associated with the currently executing @observable_source_asset, if inside the body of such an asset.

Right now it's pretty cumbersome to access, users have to use the (pretty opaque) context.asset_key_for_output("result"). Not sure if this is better addressed elsewhere w/ the context reworks.

@observable_source_asset(
    ...
)
def source_asset_with_observer(context: OpExecutionContext):
    context.log.info("Self key: %s", context.observable_asset_key)

Also tentatively updates asset_key prop to return the observable asset key.

Test Plan

New unit test.

[benpankow]

• #18898 👈 (View in Graphite)
• master

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