feat: map to g2 on BLS12-381

[ivokub]

Description

This commit cherry-picks only map-to-g2 from #1473. Depends on #1447.

Hash-to-G2 will be worked on in #1473 as requires some modification in other gadgets which could be pushed separately.

Type of change

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

How has this been tested?

• TestG2Isogeny
• TestClearCofactorTestSolve
• TestMapToCurve2
• TestMapToG2

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

[Copilot]

Copilot reviewed 34 out of 36 changed files in this pull request and generated no comments.

Files not reviewed (2)

• go.mod: Language not supported
• internal/stats/latest_stats.csv: Language not supported

Comments suppressed due to low confidence (3)

std/algebra/emulated/sw_bls12381/g2_test.go:227

• Verify that the scalar field used in tests (ecc.BN254.ScalarField()) is appropriate for BLS12-381 operations, as an inadvertent mismatch might lead to subtle errors during verification.

err := test.IsSolved(&MultiScalarMulTest{}, &assignment, ecc.BN254.ScalarField())

std/algebra/emulated/sw_bls12381/g1.go:190

• [nitpick] Consider clarifying the subgroup order check within AssertIsOnG1 with additional inline comments to improve maintainability and ease future reviews.

func (g1 *G1) AssertIsOnG1(P *G1Affine) {

std/algebra/emulated/sw_bls12381/hints.go:345

• [nitpick] Consider clarifying the function comment regarding the expected order of nativeOutputs in g1SqrtRatioHint to ensure maintainers understand the intended behavior.

func g1SqrtRatioHint(nativeMod *big.Int, nativeInputs, nativeOutputs []*big.Int) error {