RFR: 8303530: Add system property for custom JAXP configuration file [v3]

[Joe Wang]

> Add a system property, jdk.xml.config.file, to return the path to a custom JAXP configuration file. The current configuration file, jaxp.properties, that the JDK supports will become the default configuration file.
> 
> CSR: https://bugs.openjdk.org/browse/JDK-8303531
> 
> Tests: XML SQE and JCK tests passed.

Joe Wang has updated the pull request incrementally with one additional commit since the last revision:

  update config file description; add a general scope and order section; move value definition for external properties to class description

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12985/files
  - new: https://git.openjdk.org/jdk/pull/12985/files/014d99fd..43407bdc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12985&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12985&range=01-02

  Stats: 392 lines in 23 files changed: 112 ins; 175 del; 105 mod
  Patch: https://git.openjdk.org/jdk/pull/12985.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12985/head:pull/12985

PR: https://git.openjdk.org/jdk/pull/12985