default AuthStore gives NPE where XDG_RUNTIME_DIR is not present

[maxandersen]

Description of the bug

https://github.com/oras-project/oras-java/blob/main/src/main/java/land/oras/auth/AuthStore.java#L93 assumes certain env exist.

therefore basic init fails on windows and mac:

///usr/bin/env jbang "$0" "$@" ; exit $?
//DEPS land.oras:oras-java-sdk:0.2.3

import land.oras.Registry;

public class PullImage {
    public static void main(String[] args) {

            Registry registry = Registry.Builder.builder().defaults().build();
    }
} 

Gives you:

Exception in thread "main" java.lang.NullPointerException
        at java.base/java.util.Objects.requireNonNull(Objects.java:209)
        at java.base/sun.nio.fs.UnixFileSystem.getPath(UnixFileSystem.java:263)
        at java.base/java.nio.file.Path.of(Path.java:147)
        at land.oras.auth.AuthStore.newStore(AuthStore.java:93)
        at land.oras.auth.AuthStoreAuthenticationProvider.<init>(AuthStoreAuthenticationProvider.java:41)
        at land.oras.Registry$Builder.defaults(Registry.java:672)
        at PullImage.main(PullImage.java:14)

[jonesbusy]

Fixed by #255

Also enhanced a bit existing tests that would help testing when oras-project/oras-go#918 implemented on Java side

Podman/buildah support also the format repo/namespace on the auth config. Currently this is not handled by the SDK (also missing in oras-go)