Consider adding write_to_file() in PackageManifest's impl

[kayagokalp]

          It might be cleaner to add this logic in a a `write_to_file()` in `PackageManifest`'s impl, as  there is already a `from_file()` there.

Then this function could be simpler:

    pub(crate) fn update_proxy_address_in_manifest(
        address: &str,
        manifest: &PackageManifestFile,
    ) -> Result<()> {
           match manifest.proxy() {
               Ok(_) => {
                   let mut new_manifest = manifest.clone();
                   new_manifest.proxy.unwrap().address = address;
                   new_manifest.write_to_file();
                   Ok(())
               }
               Err(_) => // return error?
           }
}

Originally posted by @sdankel in #6069 (comment)