Expose a mechanism to create a new FunctionCallExpr

Many of the expression structs have public fields, which enables the AST to be rewritten with them (as a consumer can create them). The [FunctionCallExpr](https://github.com/cloudflare/wirefilter/blob/8bab3eda942b1b0a8b96d75fa961c175987a4c98/engine/src/ast/function_expr.rs#L215-L223) struct's fields are marked `pub(crate)` however. Were these left private intentionally? If yes, would you be open to exposing a way to create this struct (perhaps making any needed validation checks, etc)? If this was an oversight, would you be open to a PR making them public?

I have a use case for rewriting expressions associated with specific fields, and I think without some way to create a `FunctionCallExpr`, I will not have fully rewritten all expressions in the filter.