[foreign-jextract] RFR: 8239809: Need API to access attributes

[Henry Jen]

Add API to allow associate attributes with Declaration.

An attribute can have multiple values, thus a list of ConstantDesc.

The test case is based on [JDK-8223105](https://bugs.openjdk.java.net/browse/JDK-8223105), Windows test is simply parsing at this point, need to add validation as the header is implemented differently. 

We may want to add a more generic test case with some standard attributes.

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Commits:
 - c26d157f: Add javadoc for attributes API
 - 85fd0fe8: 8239809: Need API to access attributes

Changes: https://git.openjdk.java.net/panama-foreign/pull/33/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/33/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8239809
  Stats: 312 lines in 8 files changed: 269 ins; 7 del; 36 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/33.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/33/head:pull/33

PR: https://git.openjdk.java.net/panama-foreign/pull/33