RFR: 8303530: Add system property for custom JAXP configuration file [v7]

[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 javadoc as discussed in the review meeting; add a sample configuration file jaxp.properties; update impl and tests accordingly.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12985/files
  - new: https://git.openjdk.org/jdk/pull/12985/files/d62f58fd..f621b5d8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12985&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12985&range=05-06

  Stats: 1346 lines in 34 files changed: 1191 ins; 67 del; 88 mod
  Patch: https://git.openjdk.org/jdk/pull/12985.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12985/head:pull/12985

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