External type hints for ICU Message Format checks

### Describe the problem

Some formats (at least ARB) provide type hints for ICU Message Format externally. There is currently no way for Weblate to understand this.

```
"loadCountMoreParticipants": "{count, plural, one{Загрузить еще одного участника} few{Загрузить еще # участников} other{Загрузить еще # участников}}",
"@loadCountMoreParticipants": {
"type": "String",
"placeholders": {
"count": {
"type": "int"
}
}
},
```

### Solution brainstorm

* Introduce flag like `icu-format-hint:placeholder:type:other-placeholder:other-type` and let the ICU typing check get types from here if present
* Extend the ARB parser to emit these flags for the ARB placeholder annotations

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

* https://github.com/WeblateOrg/weblate/issues/16619