Authentication of username and password when BR backups data

## Feature Request

**Describe the feature you'd like:**

* Require username and password for access authentication when BR backups data
* TiDB Operator deployment supports this access authentication

**Describe alternatives you've considered:**

```
br [backup] [full|db|table] –tidb.user=${user_name} –tidb.password=${password}
```

Parameter explanation:
- `tidb.user`: specify the username to access the backup cluster, the default value is ”root“
- `tidb.password`: specify the password to access the backup cluster, the default value is “”

Parameter behavior:
- If the user does not specify a password on the command line, interactively prompt the user to enter the password
```
br [backup|restore|point] [full|db|table|point] –tidb.user=${user_name}
Enter password:
```
- If the user name or password is wrong, the following information will be prompted
```
br [backup|restore|point] [full|db|table] –tidb.user=${user_name} –tidb.password=${password}
Access denied for user {user_name}
```

For tidb operator deployment, configure username and password following this design [tidbaccessconfig](https://github.com/pingcap/tidb-operator/blob/v1.3.6/docs/api-references/docs.md#tidbaccessconfig).