Add (Field|Schema).withMetadata() helper to match other Arrow bindings

[kentkwu]

Describe the enhancement requested

Manipulating field and schema metadata in the JS bindings currently requires manually constructing Map<string, string> instances and re-instantiating Field/Objects objects.

Other Arrow implementations:

• C++ Field + Schema

• Rust Field + Schema

expose a helper that replaces a field/schema’s metadata map (and accepts null/None to clear it).

Request

Add a Field.withMetadata(), and Schema.withMetadata() method that:

• Accepts a Map<string, string>, a plain object Record<string, string>, or null (to clear metadata).
• Returns a new Field with the supplied metadata (replacement semantics, not merging).

This brings the JavaScript bindings in line with the existing C++/PyArrow/Rust APIs and makes metadata updates more ergonomic (and less error-prone).