AIP-76: Add PartitionAtRuntime authoring API to Task SDK

[anishgirianish]

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First of three PRs decomposing draft #65300 for AIP-76, per the design agreed in #44146.

Adds the Task SDK producer surface for runtime partition keys:

• PartitionAtRuntime() marker timetable
• PartitionKey(key, extra) for per-event metadata
• outlet_events[asset].partition_keys = [...] / .add_partition(...)
• self.partition_keys = [...] inside @asset via _AssetSelfProxy
• outlet_events kwarg injection in @asset / @asset.multi

No server-side consumption yet, that lands in PR 2 (core behavior + fan-out) along with PR 3 (example DAG + docs).

cc: @Lee-W

related: #44146
related: #65300

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