Add supports for perpetualpowersoftau SRS

[alxiong]

Relevant sources:

• ark-circom's zkey.rs parsing code
• snarkjs code for loading ppot
• PSE's repo (as a continuation of Weijie's et.al work)
• Kobi's phase2-bn254 repo

According to offline communication, ppot provides not only powers_of_g, but also powers_of_h, which is very useful in applications like PCS multiproofs.

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• use feature flags to selective enable diff SRS