Support mutation and rewriting during AST traversal

First of all, thank you for maintaining this project! From my experience, this crate is the only reliable PromQL parser available in Rust right now.

**Problem:**

Currently, [`walk_expr`](https://github.com/GreptimeTeam/promql-parser/blob/c4bebc845fc4b127dffc3392fbb9bd024770c33b/src/util/visitor.rs#L41) provides a great way to traverse the PromQL AST immutably. In the official Go implementation of the PromQL parser, traversal functions like [`Walk`](https://github.com/prometheus/prometheus/blob/d6c18a4d6292ec58da677984a2c810e35b6f1e2c/promql/parser/ast.go#L344-L368) and [`Inspect`](https://github.com/prometheus/prometheus/blob/d6c18a4d6292ec58da677984a2c810e35b6f1e2c/promql/parser/ast.go#L392-L399) naturally allow for AST mutation because Go passes pointers without strict mutability constraints.

However, `walk_expr` relies entirely on shared references (`&Expr`). This means if we need to manipulate the AST, such as injecting labels into selectors or optimizing queries, we currently lack built-in traversal patterns for mutation and restructuring.

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Here are some thoughts on how to implement this feature:

**1. Mutation**

To support modifying internal fields in-place without allocating a new tree, we can introduce an `ExprMutVisitor` trait and a `walk_expr_mut` function (which takes `&mut Expr`).

However, duplicating the traversal logic across `walk_expr` and `walk_expr_mut` creates a maintenance burden, as they easily fall out of sync when new variants are added to `Expr`. Instead, we can write a single declarative macro that defines the AST destructuring logic once and generates both the `&` and `&mut` walker functions.

**2. Rewriting**

For structural changes where we need to replace or delete entire nodes, we can introduce an `ExprRewriter` trait and a `walk_and_rewrite` function (which takes ownership of `Expr` and returns a new one).

Unlike the reference walkers, the restructuring logic here is fundamentally different, making it impractical to generate all three walking functions with a single macro. Writing a separate, complex macro just for one single function definition feels like overkill, so it would likely be better to just implement `walk_and_rewrite` manually.

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**Questions for maintainers:**

- Are you open to adding an `ExprMutVisitor` and an `ExprRewriter` trait?
- How do you feel about using a macro to deduplicate implementations?