fix(jetty12): bundle missing XML schemas in shared_sdk to prevent offline validation warnings (#107)

[ludoch]

Fixes #107.

Summary of Changes

• Bundle missing DTD/XSD schemas centralized inside shared_sdk/src/main/resources/: In Servlet 6 / EE10+, the official jakarta.servlet-api:6.0.0 and 6.1.0 dependencies stopped bundling the W3C XML schema files (XMLSchema.dtd, datatypes.dtd, xml.xsd). Placing them in shared_sdk/src/main/resources/jakarta/servlet/resources/ (and javax/servlet/resources/ for EE8) centralizes the schema definitions across the repository with zero XML duplication across project modules.
• Automatic inclusion in Production Runtime JARs: Because our core runtime modules (runtime_impl_jetty12, runtime_impl_jetty121, and local runtime modules) shade com.google.appengine:shared-sdk into their build artifacts via maven-shade-plugin, the schema resources automatically end up packaged inside the production runtime JARs (runtime-impl-jetty12.jar, runtime-impl-jetty121.jar, runtime-shared-jetty12.jar, etc.). This guarantees offline XML validation succeeds both in local development and production environments.
• Expose schemas to offline JSP precompiler and TLD scanners: In Jetty121EE11Sdk and Jetty121EE8Sdk, included appengine-local-runtime-shared.jar in getJetty121JspJars() so that offline TLD scanning and JspC precompilation during dev_appserver startup can locate the DTDs/XSDs on their isolated classloader without logging missing-schema validation warnings.