feat constant 'randomness' - domain-size-independent vk

[Tabaie]

This PR fixes (i.e. always the same value) the randomness used in unsafe KZG SRS production. This results in getting the same SRS for a given domain every time, and even the same verifying key for different domain sizes over the same curve.
The intended application of this are tests involving batch verification of commitments - for example when verifying multiple PlonK proofs.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[ivokub]

I would instead use an option for this a la WithAlpha etc. and keep the default behaviour to be randomized. I think if we by default use fixed alpha then it may obscure some other errors (e.g. some deserialized values are static when they are not supposed to be).

[gbotrel]

+1 with @ivokub suggestion, just also on the off chance that some cavalier users, by mistake, do use this SRS in prod.