RFR: 8307547: Support variant collations [v5]

[Naoto Sato]

> The fix to https://bugs.openjdk.org/browse/JDK-8306927 switched the default collation for Swedish to the modern one. In order to provide a means for users who need the old collation, this PR intends to make `Collator` recognize the `co` Unicode locale extension so that multiple implementations for a locale can be provided. I would also like reviews for the corresponding CSR.

Naoto Sato has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains ten additional commits since the last revision:

 - Explicitly mention `Swedish` in the example.
 - Merge branch 'master' into JDK-8307547-multiple-collation
 - Added "reformed" tests
 - Review comments
 - http -> https
 - Test refinement
 - Clean-up
 - javadoc
 - 8307547: Support for multiple collation tables

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13917/files
  - new: https://git.openjdk.org/jdk/pull/13917/files/4ab39664..bfb19576

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13917&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13917&range=03-04

  Stats: 74851 lines in 982 files changed: 63014 ins; 5060 del; 6777 mod
  Patch: https://git.openjdk.org/jdk/pull/13917.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13917/head:pull/13917

PR: https://git.openjdk.org/jdk/pull/13917