Extract configuration metadata from source code

[ppkarwasz]

Following the completion of #1955, as a second step towards #1954, we should:

• develop a tool that extracts the configuration options of all Log4j plugins, together with Javadoc associated to it and convert it into the chosen metadata format,
• discuss a mechanism to bundle the plugins metadata with the Maven artifacts (e.g. we might include it in META-INF).