RFR: 8301260: Add system property to toggle XML Signature secure validation mode

[Sean Mullan]

This change adds a system property that can be used to enable/disable the XML Signature secure validation mode. This is useful for enabling/disabling the mode at runtime. The system property will supersede and have the same name as the XMLCryptoContext property that can be used to enable/disable the mode: "org.jcp.xml.dsig.secureValidation".

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Commit messages:
 - Remove whitespace.
 - Initial revision.

Changes: https://git.openjdk.org/jdk/pull/12365/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12365&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301260
  Stats: 149 lines in 4 files changed: 143 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/12365.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12365/head:pull/12365

PR: https://git.openjdk.org/jdk/pull/12365