**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.
{
    std::shared_ptr<MergeTreeIndexGranuleFullText> granule
            = std::dynamic_pointer_cast<MergeTreeIndexGranuleFullText>(idx_granule);
    if (!granule)
        throw Exception(
                "BloomFilter index condition got a granule with the wrong type.", ErrorCodes::LOGICAL_ERROR);
    
    std::vector<BoolMask> rpn_stack;
    for (const auto & element : rpn)
    {
        if (element.function == RPNElement::FUNCTION_UNKNOWN)
        {
            rpn_stack.emplace_back(true, true);
        }
        else if (element.function == RPNElement::FUNCTION_EQUALS
             || element.function == RPNElement::FUNCTION_NOT_EQUALS
             || element.function == RPNElement::FUNCTION_HAS)
        {
            rpn_stack.emplace_back(granule->bloom_filters[element.key_column].contains(*element.bloom_filter), true);
            if (element.function == RPNElement::FUNCTION_NOT_EQUALS)
                rpn_stack.back() = !rpn_stack.back();