[Rust] Get type of expression

**Description of the issue**

How do I get the type of an expression in Rust?

For example, I want to constrain this to only accesses on string-like expressions.
```ql
private class IndexExprOnNonConstantString extends IndexExpr {
IndexExprOnNonConstantString() { not this.getBase() instanceof StringLiteralExpr }
}
```

**Actual**: No `getType` predicate on expr.
**Expected**: Predicate to get type of expr.

When will support be added for an - in my understanding - very basic feature?
Or is there a workaround/internal API?

首先檢視 Rust CodeQL expression model 和現有的 Expr predicates，以範例中的 getBase() 和 StringLiteralExpr 作為最初的切入點。確認是否存在內部 type API 或 workaround；當 expression type 可以被查詢到足以約束所示的 index-expression class 時，即視為完成。

由索引模型根據 Issue 內容生成。