**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.
void registerFunctionsTuple(FunctionFactory &);
void registerFunctionsMap(FunctionFactory &);
void registerFunctionsBitmap(FunctionFactory &);
void registerFunctionsBinaryRepr(FunctionFactory &);
void registerFunctionsCoding(FunctionFactory &);
void registerFunctionsCodingUUID(FunctionFactory &);
void registerFunctionChar(FunctionFactory &);
void registerFunctionsComparison(FunctionFactory &);
void registerFunctionsConditional(FunctionFactory &);
void registerFunctionsConversion(FunctionFactory &);
void registerFunctionCastOrDefault(FunctionFactory &);
void registerFunctionsDateTime(FunctionFactory &);
void registerFunctionsEmbeddedDictionaries(FunctionFactory &);
void registerFunctionsExternalDictionaries(FunctionFactory &);
void registerFunctionsExternalModels(FunctionFactory &);
void registerFunctionsFormatting(FunctionFactory &);
void registerFunctionsHashing(FunctionFactory &);
void registerFunctionsHigherOrder(FunctionFactory &);
void registerFunctionsLogical(FunctionFactory &);
void registerFunctionsMiscellaneous(FunctionFactory &);
void registerFunctionsRandom(FunctionFactory &);
void registerFunctionsReinterpret(FunctionFactory &);
void registerFunctionsRound(FunctionFactory &);
void registerFunctionsString(FunctionFactory &);
void registerFunctionsStringArray(FunctionFactory &);
void registerFunctionsStringSearch(FunctionFactory &);
void registerFunctionsStringRegexp(FunctionFactory &);
void registerFunctionsStringSimilarity(FunctionFactory &);
void registerFunctionsURL(FunctionFactory &);
void registerFunctionsVisitParam(FunctionFactory &);