RFR: 8313718: make container at requires command configurable

[Mikhailo Seledtsov]

Container ecosystem is growing. It would be beneficial to define custom command to figure out whether a specific test host or environment allows for container testing. This enhancement seeks to make the command used by jtreg "requires" extension configurable, specifically test/jtreg-ext/requires/VMProps.java checkContainerSupport().

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Commit messages:
 - Renamed docker support to container support
 - Added useful logging; updated comments
 - Allow use of u0020 unicode to escape spaces
 - 8313718: make container at requires command configurable - initial drop

Changes: https://git.openjdk.org/jdk/pull/15475/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15475&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313718
  Stats: 44 lines in 2 files changed: 26 ins; 0 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/15475.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15475/head:pull/15475

PR: https://git.openjdk.org/jdk/pull/15475