[Frontend] Online Pooling API

[DarkLight1337]

Previously, #11129 made Embeddings API unusable for reward models. This PR adds a new Pooling API that satisfies this need. It also adds backward compatibility by making Embedding API fall back to Pooling API if the model doesn't support embedding outputs (which is the case for reward models).

FIX #11446

[github-actions]

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[Isotr0py]

LGTM!