Add override for changelog path

[Gankra]

Of the 3 autoincludes (readme, license, changelog), changelogs are the only one without any support for a manual path that wins out over auto-includes. As a result, there's no way to disambiguate when you have a CHANGELOG and CHANGELOG-OTHER or whatever. We kinda just... pick one.

axoproject/src/rust.rs

Lines 250 to 256 in af87635

	readme_file: package.readme().map(|readme| package_root.join(readme)),
	license_files: package
	.license_file()
	.map(ToOwned::to_owned)
	.into_iter()
	.collect(),
	changelog_file: None,

The annoying thing here is for the other two we're relying on Cargo having a builtin notion for license/readme paths, but it has no such notion for changelogs. As a result we'll need to define our own under metadata.dist, which axoproject isn't "supposed" to read but honestly I expect that separation isn't long for the world, so whatever let's just read out that one field.