Update chain signatures example

[bowenwang1996]

After the most recent release of chain signatures https://github.com/Near-One/mpc/releases/tag/2.0.0-rc.1 there is an important change to the signing interface. It now supports multiple domains where each domain is a master key of a specific key type (currrently either ECDSA or EdDSA). The arguments have also changed. Here is a example transaction of eddsa signing using near cli on testnet.

near contract call-function as-transaction v1.signer-prod.testnet sign json-args '{"request": { "domain_id": 1, "path": "m/44'\''/60'\''/0'\''/0/0", "payload_v2": { "Eddsa": "00000a0000000000000000000000000000000000000000003000000002000000" }}}' prepaid-gas '100.0 Tgas' attached-deposit '1 yoctoNEAR' sign-as n1-multichain.testnet network-config testnet sign-with-keychain send

and the corresponding transaction can be seen here.

The old way of requesting signing still works, but in the docs we should promote the new way of signing so that new developers can be onboarded to the new format of signing requests.

[thisisjoshford]

Thanks for filing this issue @bowenwang1996 , will get this prioritized for this coming sprint cycle.

[matiasbenary]

I worked on this PR: #60