implement signArbitrary for MockKeplr

[0xLaurenzo]

Is your feature request related to a problem? Please describe.
Currently, MockKeplr supports signDirect and signAmino, but throws a "Not implemented" error for signArbitrary and verifyArbitrary, making the testing of certain features that want to use adr-36 more difficult, especially in CI/CD environments

Describe the solution you'd like
Implement signArbitrary and verifyArbitrary for MockKeplr

Describe alternatives you've considered
Inlining signArbitrary and signing using MockKeplr through signAmino does not work due to type constraints. signArbitrary internally calls appears to call serializeSignDoc internally. This behaviour is also seen in the verification functions of verifyADR36AminoSignDoc

Additional context
I believe most code can be copy pasted from pre-existing adr36 code already within keplr, such as the verification funtions

[0xLaurenzo]

I believe this would almost completely suffice: 222bd16, except for the bech32 prefix in the verification function

[HeesungB]

@LaurensKubat Hello. I'll check about it! Thank you.

[HeesungB]

Hello.

It would be nice if it could be changed to something like this, please check.

import {
  encodeSecp256k1Pubkey,
  makeADR36AminoSignDoc,
  verifyADR36Amino,
} from "@keplr-wallet/cosmos";

async signArbitrary(
    chainId: string,
    signer: string,
    data: string | Uint8Array
  ): Promise<StdSignature> {
    const wallet = await this.getWallet(chainId);

    const key = await this.getKey(chainId);
    if (signer !== key.bech32Address) {
      throw new Error("Unmatched signer");
    }

    const signature = wallet.signDigest32(
      Hash.sha256(serializeSignDoc(makeADR36AminoSignDoc(signer, data)))
    );

    return {
      pub_key: encodeSecp256k1Pubkey(wallet.getPubKey().toBytes()),
      signature: encodeSecp256k1Signature(
        wallet.getPubKey().toBytes(),
        new Uint8Array([...signature.r, ...signature.s])
      ).signature,
    };
  }

  async verifyArbitrary(
    chainId: string,
    signer: string,
    data: string | Uint8Array,
    signature: StdSignature
  ): Promise<boolean> {
    const wallet = await this.getWallet(chainId);

    const key = await this.getKey(chainId);
    if (signer !== key.bech32Address) {
      throw new Error("Unmatched signer");
    }

    return verifyADR36Amino(
      this.chainInfos.find((c) => c.chainId === chainId)!.bech32Config
        .bech32PrefixAccAddr,
      signer,
      data,
      wallet.getPubKey().toBytes(),
      Buffer.from(signature.signature, "base64")
    );
  }