RFR: 8225181: KeyStore should have a getAttributes method [v3]

[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.

Weijun Wang has updated the pull request incrementally with one additional commit since the last revision:

  more clear and precise spec

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6026/files
  - new: https://git.openjdk.java.net/jdk/pull/6026/files/7238e784..aaaa6145

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6026&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6026&range=01-02

  Stats: 16 lines in 2 files changed: 4 ins; 0 del; 12 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