[RFC] common, server : add top-a sampler

[Artefact2]

Top-A sampling is an interesting sampling technique that behaves like a dynamic Min-P. More details here: https://github.com/BlinkDL/RWKV-LM/tree/4cb363e5aa31978d801a47bc89d28e927ab6912e#the-top-a-sampling-method

When the most likely token has a high probability, this behaves like min-p with high P. When the most likely token has a low probability, this behaves like min-p with low P. The a parameter selects the cutoff point (a*P^2 where P is the probability of the most likely token).

This commit adds top-a sampling to llama.cpp and server.cpp, which improves compatibility with clients like AI Horde. Because this sampler works very similarly to min-p, adding it is mostly free in terms of actual code changes.