Add nano_v3 vLLM reasoning parser plugin

[dpickem]

vLLM reasoning-parser plugin for the Nemotron Nano v3 reasoning format, needed when serving Nano v3.5 with --reasoning-parser nano_v3 for reward profiling.

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[macandro96]

We're adding the ability to support custom parsers and also add nano-v3 reasoning parser here: #2569