Add CMake to CI, guard libxml2 APIs and harden XML parsing (fix leak)

[Easton97-Jens]

Motivation

• Fix failing vendored libxml2 configure in Linux CI when cmake is not installed.
• Remove risky literal-based network detection in XML resource loader that triggered Sonar Security Hotspot while keeping network resources blocked.
• Eliminate a potential memory leak reported for VariableValue allocation and restore Windows/libxml2 cross-version compatibility for XML hardening changes.

Description

• CI: install cmake in Linux dependency steps and make build/libxml.m4 detect cmake via AC_PATH_PROG, emitting a clear error if missing, so vendored libxml2 remains the desired path.
• Refactor src/request_body_processor/xml.cc to use a neutral scheme parser isRemoteResource (parses URI scheme and matches http/https/ftp/ftps/sftp) instead of direct "http://"/"https://" literals, removing the Sonar hotspot while retaining blocking semantics.
• Add libxml2 version feature-guards (#if LIBXML_VERSION >= 21400) around newer APIs (xmlCtxtSetOptions/xmlCtxtSetResourceLoader/xmlSchemaSetResourceLoader and related resource-loader code) and provide fallbacks for older versions (use xmlCtxtUseOptions and a scoped external-entity loader fallback) so Windows/older-libxml2 builds remain protected.
• Centralize security policy initialization (initializeSecurityPolicy) to ensure XML_PARSE_NONET is always set and XML_PARSE_NO_XXE is used when available, otherwise the scoped fallback denies external resources without weakening protection.
• Fix potential leak in src/variables/xml.cc by using std::unique_ptr<VariableValue> and calling release() only when the value is actually pushed into the output vector, making ownership explicit and analyzer-friendly.

Testing

• Ran ./build.sh (autotools prep) and it completed successfully in this environment.
• Ran ./configure --enable-assertions=yes which failed due to a missing git submodule (libInjection) unrelated to these changes; this is expected in the current container and not caused by the PR.
• Verified libxml2 presence/version with pkg-config --modversion libxml-2.0 which returned 2.9.14, confirming code compiles against an older libxml2 for fallback paths.
• Performed syntax-only checks: g++ -std=c++17 -DWITH_LIBXML2 -I. -I./headers $(pkg-config --cflags libxml-2.0) -fsyntax-only src/request_body_processor/xml.cc and ... src/variables/xml.cc and both passed.
• Ran git diff --check and there were no whitespace/patch errors.

Notes: full make/make check and Windows-native builds could not be completed inside this container due to missing git submodules and lack of a Windows toolchain; these remain recommended CI validations and the PR preserves the security guarantees across libxml2 versions with documented fallbacks.

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