Add a fuzz test for Sign+Verify

I attempted to do this, but in https://github.com/smartcontractkit/chainlink-starknet/pull/163 but it's hard to filter out invalid reports. We'd need to run SplitFelt on the report and validate that each segment is a valid felt.

//go:build go1.18
// +build go1.18

package keys

import (
	cryptorand "crypto/rand"
	"encoding/hex"
	"testing"

	"github.com/smartcontractkit/chainlink-starknet/relayer/pkg/chainlink/ocr2/medianreport"
	ocrtypes "github.com/smartcontractkit/libocr/offchainreporting2/types"
	"github.com/stretchr/testify/require"
)

// go test -tags=go1.18 -fuzz ./...
func FuzzStarkNetKeyring_Sign_Verify(f *testing.F) {
	kr1, err := NewOCR2Key(cryptorand.Reader)
	require.NoError(f, err)
	kr2, err := NewOCR2Key(cryptorand.Reader)
	require.NoError(f, err)

	digest := "00044e5d4f35325e464c87374b13c512f60e09d1236dd902f4bef4c9aedd7300"
	bytes, err := hex.DecodeString(digest)
	require.NoError(f, err)
	configDigest, err := ocrtypes.BytesToConfigDigest(bytes)
	require.NoError(f, err)

	ctx := ocrtypes.ReportContext{
		ReportTimestamp: ocrtypes.ReportTimestamp{
			ConfigDigest: configDigest,
			Epoch:        1,
			Round:        1,
		},
		ExtraHash: [32]byte{
			255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
			255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
		},
	}
	report := ocrtypes.Report{
		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 97, 91, 43, 83, // observations_timestamp
		0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // observers
		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, // len
		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 73, 150, 2, 210, // observation 1
		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 73, 150, 2, 211, // observation 2
		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 13, 224, 182, 179, 167, 100, 0, 0, // juels per fee coin (1 with 18 decimal places)
	}

	// Seed with valid report
	f.Add([]byte(report))
	f.Fuzz(func(t *testing.T, report []byte) {
		cdc := medianreport.ReportCodec{}
		// skip invalid reports
		if _, err := cdc.MedianFromReport(report); err != nil {
			t.Skip()
		}
		if _, err := ReportToSigData(ctx, report); err != nil {
			t.Skip()
		}
		sig, err := kr1.Sign(ctx, report)
		require.NoError(t, err)
		result := kr2.Verify(kr1.PublicKey(), ctx, report, sig)
		require.True(t, result)
	})
}

Start in the keys package with FuzzStarkNetKeyring_Sign_Verify, ReportToSigData, Sign, and Verify, then inspect medianreport.ReportCodec.MedianFromReport and SplitFelt. Run the fuzz command shown in the issue. Done means valid reports are filtered by felt-segment validation and valid inputs consistently sign and verify successfully.