feat: added HashToG2 circuit for BLS12-381

[weijiguo]

Description

This PR exposes the g2SqrtRatio function as G2SqrtRatio so that it could be used as a hint in gnark to calculate the HashToG2 circuit for BLS12-381.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How has this been tested?

• Tested in the gnark library as part of the HashToG2 function for BLS12-381.

How has this been benchmarked?

No.

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

[CLAassistant]

All committers have signed the CLA.

[weijiguo]

This PR is part of efforts to address Consensys/gnark#648