RFR: 8372661: Add a null-safe static factory method to "jdk.test.lib.net.SimpleSSLContext" [v4]

[Volkan Yazici]

> Overhauls `SimpleSSLContext` to remove the need for null checks at the call site, and to accept a key store file search path, which removes the need to copy-paste `SimpleSSLContext` just to change the search path.

Volkan Yazici has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Make sure `get()` returns the same instance

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28765/files
  - new: https://git.openjdk.org/jdk/pull/28765/files/b52e442f..fa00d901

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28765&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28765&range=02-03

  Stats: 10 lines in 1 file changed: 0 ins; 8 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28765.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28765/head:pull/28765

PR: https://git.openjdk.org/jdk/pull/28765