One of the big improvements added in Go 1.24 is `omitzero`[0][1][2],
which makes it possible to omit the "zero value" of a given Go type when
marshaling to JSON, or if the `IsZero() bool` function on the type
returns `true`.

To make it possible for `oapi-codegen` to use this, we can wire in a new
extension, `x-omitzero` which allows per-field tuning of this behaviour.

This is something that is safe to generate alongside the `omitempty`
tags[3]:

> If both "omitempty" and "omitzero" are specified, the field will be
omitted if the value is either empty or zero (or both).

To validate this, we can add a new example for the extension, which
provides testing of the expected behaviour.

As this requires Go 1.24, we need to set up the boilerplate for a new
module that only runs Go 1.24.

Closes oapi-codegen#1899.

[0]: https://www.jvt.me/posts/2025/02/12/go-omitzero-124/
[1]: https://www.bytesizego.com/blog/go-124-omitzero
[2]: https://antonz.org/go-1-24/#omit-zero-values-in-json
[3]: https://pkg.go.dev/encoding/json