**Use case**

I want to estimate better parameters for fulltext (`ngrambf_v1`) index.

**Describe the solution you'd like**

Implement it as SQL function:

`ngrams(string, N)`

that returns array of FixedString.

**Describe alternatives you've considered**

It can be done with less efficient and more cumbersome way:

```
WITH 3 AS n, extractAll(text, '.') AS chars 
SELECT 
  arrayMap(x -> arrayStringConcat(x), 
    arrayFilter(x -> length(x) = n, 
      arrayMap(x, i -> arraySlice(chars, i, n), 
        chars, arrayEnumerate(chars)))) AS ngrams
```

**Additional context**

`ngramsUTF8` can also be implemented to extract ngrams of Unicode code points.
{
                return false;
            }
        }
    }
    if (!key_exists && !map_key_exists)
        return false;
    if (map_key_exists && (function_name == "has" || function_name == "mapContains"))
    {
        out.key_column = key_column_num;
        out.function = RPNElement::FUNCTION_HAS;
        out.bloom_filter = std::make_unique<BloomFilter>(params);
        stringToBloomFilter(const_value.get<String>(), token_extractor, *out.bloom_filter);
        return true;
    }
    else if (function_name == "has")
    {
        out.key_column = key_column_num;
        out.function = RPNElement::FUNCTION_HAS;
        out.bloom_filter = std::make_unique<BloomFilter>(params);
        stringToBloomFilter(const_value.get<String>(), token_extractor, *out.bloom_filter);
        return true;
    }
    if (function_name == "notEquals")
    {
        out.key_column = key_column_num;
        out.function = RPNElement::FUNCTION_NOT_EQUALS;
        out.bloom_filter = std::make_unique<BloomFilter>(params);
        stringToBloomFilter(const_value.get<String>(), token_extractor, *out.bloom_filter);
        return true;