[V1] [Spec Decode] Support random sampling in Rejection Sampler

[LiuXiaoxuanPKU]

This PR tries to:

1. Support random sampling in rejection sampler. This should be general to different drafting methods, not limited to ngram spec decode.
2. Clean up and reuse rejection sampling tests from V0.

This PR does not:

1. Change model runner to use rejection sampler with random sampling. We need one extra PR to support ngram with random sampling.

Current status:
Still under testing for correctness.

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