RFR: 8225181: KeyStore should have a getAttributes method [v2]

[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:

  clarification on protected attributes

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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/86bbe87b..7238e784

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6026&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6026&range=00-01

  Stats: 18 lines in 2 files changed: 13 ins; 0 del; 5 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