Releasing 2.1.0

[poikilotherm]

Some TODOs for this repo that are missing before we can release 2.1.0:

• Create examples with the new annotations to publish on the website
• Refactor README
• Refactor Javadocs on the Exporters
• Refactor all the relevant Javadoc on the new plugin metadata / loading code, making the Javadoc website a lot better
• Add package-info.java everywhere with as much explanations as possible
• Add an integration test for the plugin loader that tries end to end loading a plugin with contracts non-existant in the code
• Try to find ways how one might try to use the plugin system to execute malicious code
• Finalize decision about requiring installations to update their installed plugins with newer versions, which the plugin authors will need to provide. Technically, this would be a breaking change from an admin and plugin author perspective. The code currently is strict about this and will not allow plugins to be loaded if metadata is missing.

[poikilotherm]

Also, the 2.1.0-SNAPSHOT should be integrated and tested in both https://github.com/gdcc/dataverse-exporters and Dataverse upstream. (In upstream it will require updates to the in-tree exporter plugins due to the new loader as well as the changes to the export data provider.)

[poikilotherm]

Some more ideas for checks and optimizations:

• Does the processor currently check and enforce an API_LEVEL is >0?
• Do not continue loading plugins when there are 0 plugins after pre-loading around. Also, having 0 plugins after loading currently triggers warnings - this should probably be a quiet event, depending on "no sources around" and "sources present, but failed to load".