V2: Optional client-side proving feature flag

[ethenotethan]

V2 supports both disperser/client side commitment generation. I.e a user can either:

1. Have the disperser generate the respective polynomial commitments for some blob to give to client who injects values into blob header OR
2. Have the client use their own kzg prover to generate the commitments and inject into blob header without relying on disperser

There should exist some flag (e.g, EIGENDA_V2_USE_DISPERSER_PROVER) that allows a user to type filter between generation modes when running the service.