Dictionary FixedString as key column data type.

**Use case**

Ability to use FixedString in dictionary key column.

**Describe the solution you'd like**

```
CREATE DICTIONARY dict (
key FixedString(3),
....
)
PRIMARY KEY key
....

There was an error on xxx: Code: 50. DB::Exception: Unknown type FixedString(3) for dictionary attribute.
```

**Describe alternatives you've considered**

Use String instead, but may be it's less efficient.

**Additional context**

> Add any other context or screenshots about the feature request here.