External type hints for ICU Message Format checks

[nijel]

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

• ICU check returns false error result on int placeholder for plural #16619