RFR: 8225181: KeyStore should have a getAttributes method

[Weijun Wang]

Add `KeyStore::getAttributes` so that one can get the attributes of an entry without retrieving the entry first. This is especially useful for a private key entry which can only be retrieved with a password.

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Commit messages:
 - 8225181: KeyStore should have a getAttributes method

Changes: https://git.openjdk.java.net/jdk/pull/6026/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6026&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8225181
  Stats: 152 lines in 6 files changed: 150 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6026.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6026/head:pull/6026

PR: https://git.openjdk.java.net/jdk/pull/6026