add r1 signature

[weixuefeng]

No description provided.

[ludowkm]

hi @weixuefeng , thank you for your PR.
Can you describe detail about this new part please

[weixuefeng]

i used newchain for sign message, newchain used secp256r1 algorithm and i had add signature r1 for eth-sig-util(dart).

i supply this merge for review code.
thanks for reply. @ludowkm

[juampiq6]

sorry but i think changing all the code with ifs, just to support one more elliptic curve, is not worth it. Maybe we should think about creating a EthSigUtil class which doesnt have static methods and which you can initialize with an specific curve to use parameter