Verify signature

[Hirama]

Added VerifyTransactionSignature function:

• Verifies the signature of a SignedTransaction.
• Uses Borsh serialization and SHA-256 hashing.
• Supports ED25519 key type for verification.

Example usage:

import (
    "fmt"
    "github.com/aurora-is-near/near-api-go/near"
)

func main() {
    var signedTx *near.SignedTransaction
    // Initialize signedTx...

    isValid, err := near.VerifyTransactionSignature(signedTx)
    if err != nil {
        fmt.Println("Error:", err)
    } else if isValid {
        fmt.Println("Signature is valid.")
    } else {
        fmt.Println("Signature is invalid.")
    }
}