Core decryptFile + encryptFile methods

[tomholub]

part of #275, do before #195 or #196

need to add the following method to Core. Roughly:

    public func decryptFile(encrypted: Data, keys: [PrvKeyInfo], msgPwd: String?) throws -> CoreRes.DecryptFile {
                let json: [String : Any?]? = ["keys": try keys.map { try $0.toDict() }, "msgPwd": msgPwd]
                let decrypted = try call("decryptFile", jsonDict: json, data: encrypted)
                let meta = try decrypted.json.decodeJson(as: CoreRes.DecryptFileMeta.self)
                return CoreRes.DecryptFile(...) 
     }

here is the code of the typescript handler:

  public decryptFile = async (uncheckedReq: any, data: Buffers): Promise<Buffers> => {
    const { keys: kisWithPp, msgPwd } = ValidateInput.decryptFile(uncheckedReq);
    const decryptedMeta = await PgpMsg.decrypt({ kisWithPp, encryptedData: Buf.concat(data), msgPwd });
    if (!decryptedMeta.success) {
      decryptedMeta.message = undefined;
      return fmtRes(decryptedMeta);
    }
    return fmtRes({ success: true, name: decryptedMeta.filename || '' }, decryptedMeta.content);
  }

need to be mindful of ability to handle the error scenario.

---

similarly for encryptFile:

  public encryptFile = async (uncheckedReq: any, data: Buffers): Promise<Buffers> => {
    const req = ValidateInput.encryptFile(uncheckedReq);
    const encrypted = await PgpMsg.encrypt({ pubkeys: req.pubKeys, data: Buf.concat(data), filename: req.name, armor: false }) as OpenPGP.EncryptBinaryResult;
    return fmtRes({}, encrypted.message.packets.write());
  }

[tomholub]

@ekievsky please get started on sending/downloading encrypted messages.

Please ensure that it's done the same way browser extension does it, and is compatible. Ask questions if unsure.

[tomholub]

To test these methods, you can write a unit test that encrypts some file, then decrypts it, then compares content.

[ekievsky]

How may I mock this case?

if (!decryptedMeta.success) {
      decryptedMeta.message = undefined;
      return fmtRes(decryptedMeta);
}