[Feature Request] Add a way to set a charset for AsyncMysqlConnection

### HHVM Version

```
HipHop VM 4.25.0-dev (rel)
Compiler: 1569642289_066482361
Repo schema: 2e5c070e835e509e06702f06ee483786e2bb4a6b
And below
```

### Operating System and Version

Ubuntu 18.04

### Standalone code, or other way to reproduce the problem

```HACK
$conn = await AsyncMysqlClientConnect(...$args);
$result = await $conn->query('SHOW GLOBAL VARIABLES LIKE 'character%');
var_dump($result->mapRowsTyped());
```

### Actual result

_latin1_ is used, instead of `character_set_client`'s value for the client's charset.

### Expected result

_utf8mb4_ (in our case) to be used.

### Desired result

Either implicitly pick up the charset or add a method to `AsyncMysqlConnectionOptions` to explicitly set the charset.