RFR: 8343881: java.lang.classfile.Attribute attributeName() method should return Utf8Entry

[Adam Sotona]

Class-File API usually expose names in a form of `java.lang.classfile.constantpool.Utf8Entry`. Unfortunately `java.lang.classfile.Attribute::attributeName` method was omitted during the API review and it returns inflated `String` value of the name.

This patch changes return type of `java.lang.classfile.Attribute::attributeName` from `String` to `java.lang.classfile.constantpool.Utf8Entry`.

Please review.

Thanks,
Adam

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Commit messages:
 - suggested patch
 - Fixed AbstractAttributeMapper and typo in UnboundAttribute
 - Reversion of AttributeMapper changes
 - 8343881: java.lang.classfile.Attribute attributeName() method should return Utf8Entry

Changes: https://git.openjdk.org/jdk/pull/22107/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22107&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343881
  Stats: 399 lines in 18 files changed: 375 ins; 5 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/22107.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22107/head:pull/22107

PR: https://git.openjdk.org/jdk/pull/22107