RFR: 8313718: make container at requires command configurable [v2]

[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().

Mikhailo Seledtsov has updated the pull request incrementally with one additional commit since the last revision:

  Fixed initialization issue

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15475/files
  - new: https://git.openjdk.org/jdk/pull/15475/files/3605d604..7ad1371a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15475&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15475&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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