Support loading the xml configuration from a read-only filesystem

[groldan]

Refactor XMLFileResourceProvider to support read-only configuration directories.

Enhances XMLFileResourceProvider to allow GeoWebCache to start with a read-only configuration directory and geowebcache.xml file. Previously, the provider assumed that the configuration directory was always writable, which caused startup failures when running in environments with restricted file permissions.

• Support read-only configuration directories:

  • hasInput() now correctly returns true if the configuration file exists or if a template is available and the directory is writable.
  • hasOutput() ensures that the configuration file can be written before allowing output operations.
  • Improved error handling when checking directory and file writability.

• Refactored file existence checks:

  • Consolidated logic for checking configuration file existence and writability into findConfigFile().
  • Moved the directory writability check from findConfigFile() to findOrCreateConfFile() to prevent premature failures.

• Improved logging and exception handling:

  • hasInput() and hasOutput() now log warnings when encountering errors instead of failing silently.
  • findOrCreateConfFile() now explicitly throws an exception when it cannot create a new configuration file due to permission issues.

• GeoWebCache can now start successfully with a pre-existing, read-only configuration directory.

• Configurations stored in a read-only filesystem (e.g., mounted from a read-only volume) can still be used without requiring write access.

• The provider only attempts to create a configuration file when explicitly required, eliminating side effects from query methods hasInput() and hasOutput().

This change improves the resilience of GeoWebCache in constrained environments while maintaining backward compatibility for writable configurations.

[aaime]

Looks good to me