RFR: 8321274: Rename ZipEntry.extraAttributes to ZipEntry.externalAttributes

[Eirik Bjorsnos]

Please consider this PR which suggests we rename `ZipEntry.extraAttributes` to `ZipEntry.externalAttributes`.

This field was introduced in [JDK-8218021](https://bugs.openjdk.org/browse/JDK-8218021), originally under the name `ZipEntry.posixPerms`. [JDK-8250968](https://bugs.openjdk.org/browse/JDK-8250968) later renamed the field to `ZipEntry.extraAttributes` and extended its semantics to hold the full two-byte value of the `external file attributes` field, as defined by `APPNOTE.TXT`

The name `extraAttributes` is misleading. It has nothing to do with the `extra field` (an unrelated structure defined in `APPNOTE.TXT`), although the name indicates it does.

To prevent confusion and make life easier for future maintainers, I suggest we rename this field to `ZipEntry.externalAttributes` and update related methods, parameters and comments accordingly.

While this change is a straightforward renaming, reviewers should consider whether it carries its weight, especially considering it might complicate future backports. 

As a note to reviewers, this PR includes the following intended updates:

- Rename `ZipEntry.extraAttributes` and any references to this field to `ZipEntry.externalAttributes`
- Update `JavaUtilZipFileAccess` to similarly rename methods to `setExternalAttributes` and `getExternalAttributes` 
- Rename the field `JarSigner.extraAttrsDetected` to `JarSigner.externalAttrsDetected`
- Rename a local variable in `JarSigner.writeEntry` to `externalAttrs`
- Rename `s.s.t.jarsigner.Main.extraAttrsDetected` to `externalAttrsDetected`
- Rename resource string key names in `s.s.t.jarsigner.Resources` from `extra.attributes.detected` to `external.attributes.detected`
- Rename method `SymlinkTest.verifyExtraAttrs` to `verifyExternalAttrs`, also updated two references to 'extra attributes' in this method
- Updated copyright in all affected files

If the resource file changes should be dropped and instead handled via `msgdop` updates, let me know and I can revert the non-default files.

I did a search across the code base to find 'extraAttrs', 'extra.attr' after these updates, and found nothing related to zip/jar. The `zip` and `jar` tests run clean:


make test TEST="test/jdk/java/util/jar"
make test TEST="test/jdk/java/util/zip"

-------------

Commit messages:
 - Rename ZipEntry.extraAttributes to ZipEntry.externalAttributes

Changes: https://git.openjdk.org/jdk/pull/16952/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16952&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321274
  Stats: 34 lines in 11 files changed: 0 ins; 0 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/16952.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16952/head:pull/16952

PR: https://git.openjdk.org/jdk/pull/16952