not support for substrate chain about snap

[Nick-1979]

@BurnWW do you have any suggestions how to get RPC urls to support more chains?

[BurnWW]

@Nick-1979
I took the time to validate the feasibility of my idea, and I hope that instead of connecting to blockchain networks within Snap, we can directly use metadata for signing.
This is the code that I have successfully verified.

packages/snap/src/rpc/signJSON.ts

import { SignerPayloadJSON } from '@polkadot/types/types';
import type { SignerResult } from '@polkadot/api/types';
import { getApi } from '../util/getApi';
import { getKeyPair, getKeyPairFromMetadata } from '../util/getKeyPair';
import { checkAndUpdateMetaData, getSavedMeta, showConfirmTx } from '.';
import { metadataExpand } from '@polkadot/extension-chains';

export const signJSON = async (
  origin: string,
  payload: SignerPayloadJSON,
): Promise<SignerResult | undefined> => {
  try {
    const def = await getSavedMeta(payload.genesisHash);
    if (def) {
      const chain = metadataExpand(def!, false)
      chain.registry.signedExtensions

      const registry = chain.registry;
      registry.setSignedExtensions(payload.signedExtensions, chain.definition.userExtensions);
      const keyPair = await getKeyPairFromMetadata(chain);

      const { signature } = registry
        .createType('ExtrinsicPayload', payload, { version: payload.version })
        .sign(keyPair);

      return { id: 1, signature };
    }

    ......
  } catch (e) {
    console.info('Error while signing JSON:', e);
    return undefined;
  }
};

export const getKeyPairFromMetadata = async (
  chain: Chain
): Promise<KeyringPair> => {
  const prefix = chain.ss58Format

  const BIP44CoinNode = (await snap.request({
    method: 'snap_getBip44Entropy',
    params: {
      coinType: DEFAULT_COIN_TYPE,
    },
  })) as JsonBIP44CoinTypeNode;

  const seed = BIP44CoinNode?.privateKey?.slice(0, 32);
  const keyring = new Keyring({ ss58Format: prefix });
  const keyPair = keyring.addFromSeed(stringToU8a(seed));

  return keyPair;
};