Add plugin support

[clausnagel]

This PR adds support for user-defined plugins to citydb-tool.

The plugin mechanism is defined in the new module citydb-plugin:

• A plugin can be defined anywhere and must extend org.citydb.plugin.Plugin.
• A plugin implements a set of extensions. Extensions can be defined by any module including even user-defined modules. The meaning and purpose of the extension is defined by the module (e.g., a "CLI extension" to add user-defined CLI commands or an "import extension" to preprocess features before importing them). All extensions must implement the interface org.citydb.plugin.Extension.
• A plugin should provide metadata through the "plugin.json" file
• The PluginManager takes care of dynamically loading plugins at runtime and provides access to all loaded plugins, extensions, and the ClassLoader used for loading the plugins.

Besides this general plugin mechanism, the citydb-cli module has been extended by this PR:

• When starting citydb-tool, all plugins are loaded by citydb-cli using the PluginManager. Plugins are recursively searched for in the plugins directory (default) or in any other directory defined by the new --plugins parameter.
• Plugins can be enabled and disabled through the new --use-plugins parameter.
• The citydb-cli module defines three extensions that allow extending the citydb-tool CLI:
  • MainCommand adds a new main command to the CLI.
  • ImportFormatCommand adds a new subcommand to the import command to add support for another data format.
  • ExportFormatCommand adds a new subcommand to the export command to add support for another data format.

More extensions to other modules will be added after merging this PR. Note: All user-defined extensions to citydb-tool will be plugins. There is no distinction between plugins and ADE extensions like in the Importer/Exporter anymore.

[yaozhihang]

Nice module. Only one comment: should we create an empty plugin folder in the CLI tool build package?

[clausnagel]

Nice module. Only one comment: should we create an empty plugin folder in the CLI tool build package?

Yes, absolutely!

[clausnagel]

added empty plugins directory with 1c39dc09112e4ff5a972905ce67e067eb7d4a72d