RFR: 8347949: Currency method to stream available Currencies

[Justin Lu]

Please review this PR and CSR which adds a method to _java.util.Currency_ which returns a stream of the available Currencies.

The motivation can be found in the CSR.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/23165/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23165&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347949
  Stats: 77 lines in 2 files changed: 69 ins; 3 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/23165.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23165/head:pull/23165

PR: https://git.openjdk.org/jdk/pull/23165