RFR: 8303530: Add system property for custom JAXP configuration file [v9]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains ten additional commits since the last revision:

 - Merge branch 'master' into JDK-8303530
 - Update based on the review meeting on 4/5. Added (duplicated) definitions in jaxp.properties; Rewrote Property Precedence with samples; Other updates.
 - update javadoc as discussed in the review meeting; add a sample configuration file jaxp.properties; update impl and tests accordingly.
 - continue support stax.properties at the impl level, though dropped from the spec
 - change prefix from jdk to java
 - change prefix from jdk to java; rm zip file that accidentally checked in
 - update config file description; add a general scope and order section; move value definition for external properties to class description
 - clean up tests; fix copy&paste error.
 - 8303530: Add system property for custom JAXP configuration file

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12985/files
  - new: https://git.openjdk.org/jdk/pull/12985/files/98a312cd..5f889bb9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12985&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12985&range=07-08

  Stats: 417343 lines in 4246 files changed: 324482 ins; 62120 del; 30741 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