Add mla-backend argument

[Fridge003]

Motivation

Followup of #5005, this PR adds --mla-backend argument to better handle the difference between normal attention backend and mla backend. In default, --attention-backend is set to flashinfer, and --mla-backend is set to triton.
Arguments like --enable-flashinfer-mla and --enable-flashmla are still usable.

Modifications

Checklist

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