- New PdpEngineConfiguration class constructor parameter - XML namespace prefix-to-URI mappings - to help fix the issue authzforce/server#66 .
- Fix CVE-2020-36518 affecting jackson dependency
- Upgrade authzforce-ce-core-pdp-api to 21.2.0

- New PdpEngineConfiguration class constructor parameter - XML namespace prefix-to-URI mappings - to help fix the issue authzforce/server#66 .
- Fix CVE-2020-36518 affecting jackson dependency
- Upgrade authzforce-ce-core-pdp-api to 21.2.0

- Fixed bug when having multiple XPath expressions (e.g. AttributeSelectors) with XPath variables in the same Policy (the list of declared variables on Saxon XPathCompiler is internally saved and not reinitialized after each call to `XPathCompiler#compile(String)` having side effects when reusing the same XPathCompiler instance).

  - `spif2xacml-for-xpath-1.0.xsl`: SPIF-to-XACML policy transformation XSLT using XPath 1.0, more verbose and less efficient than the XPath 2.0 version below.
  - `spif2xacml-for-xpath-2.0.xsl`: SPIF-to-XACML policy transformation XSLT using XPath 2.0 features (not available in 1.0), with the option to enable AuthzForce optimizations (XSLT parameter `authzforce_optimized`) for further enhancements.

- Used generated policies from sample ACME SPIF with above stylesheets to custom pdp tests XPath2.0 and XacmlVariablesAsXPathVariables.

- Replaced occurrences of XPathCompiler class in method parameters with Optional<XPathCompilerProxy>, XPathCompilerProxy being a new immutable class wrapping XPathCompiler (for safer code and fixing Spotbugs issues) from authzforce-ce-core-pdp-api, and the use of Optional to indicate it is used only if XPath is enabled (by PDP configuration or Policy(Set)Defaults/XPathVersion in enclosing Policy(Set) )
- Added feature: XPath variables in AttributeSelectors' and `xPathExpression` `AttributeValues`s' XPath expressions can now be defined by XACML VariableDefinitions (variable name used as XACML VariableId), which means XACML Variables can be used as XPath variables there. And unit tests for it based on STANAG 4774.1
- Fixed Spotbugs issues
- VariableReference interface: added getXPathVariableName(): QName
- Added unit tests for XPath 2.0 expressions in AttributeSelectors

- Added SPIF-to-XACML XSLT stylesheet

API change: Request pre-processor extension interface changed: `DecisionRequestPreprocessor.Factory#getInstance(...)` method changed: `xmlProcessor` arg removed

- authzforce-ce-core-pdp-api upgraded to 19.1.0
- PMD / Spotbugs issue fixes (New class ImmutableAttributeDesignator)
- pdp.xsd: 'version' changed from required to optional with default value equal to xsd version
- Removed BasePdpdEngine constructor arg: xacmlExpressionFactory
- Removed RootPolicyEvaluators.Base constructor arg: xacmlExpressionFactory
- Removed PdpEngineConfiguration#getXacmlExpressionFactory()
- Maven POM: pdp.xsd classes generated with immutable-xjc-plugin instead of jaxb2-value-constructor