Add flags for 3.0 testing

[cmmarslender]

Note

Low Risk
Limited to CLI/network-constants generation and gated behind optional flags; main risk is producing incorrect testnet configs if values are mis-specified.

Overview
network generate now supports additional optional inputs to populate 3.0/hard fork 2-related fields in the generated NetworkConstants (hard fork 2 height, plot filter v2 zero-bit requirement, plot v1 phase-out epoch bits, and up to three plot filter v2 adjustment heights).

The command adds corresponding Cobra flags and Viper bindings, and only sets these constants when the flags are explicitly provided (preserving prior output by default).

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