Dependencies of two different plugins can conflict

[hyfen]

Right now, plugins aren't dependencies of chronicle-etl and we just check for which chronicle-* gems are available at runtime and try to load them all. The problem is that plugins can have conflicting dependencies.

A typical scenario: for two platforms foo.com and bar.com, the ruby client libraries ruby-foo and ruby-bar will require different versions of faraday. If a user runs gem install chronicle-foo and gem install chronicle-bar and then tries to use them both within chronicle-etl, we'll get a Gem::ConflictError.

Possible solutions:

• Make this project a monolith. We centralize plugins into the chronicle-etl repo and let bundler handle dependency resolution and accept that our bundle size will be huge and potentially hard to install if any plugins require native extensions.
• Only allow one plugin to be used at the same time. Because we won't load all available plugins, the Connector Registry won't know which connectors are available (because the register_connector macro won't be called) so we'll need another technique
• Adding commonly-used plugins as dependencies in chronicle-etl.gemspec and we can guarantee that at least those ones can run well together.

In any case, it'll be important to provide enough infrastructure in chronicle-etl so that plugins will need only a few additional dependencies.

[hyfen]

Right now, the connector Registry needs a plugin to be loaded to learn which connectors are available (via the #register_connector macro called in connectors).

Instead, we could use a plugin's chronicle-PLUGIN.gemspec's metadata hash to list which connectors are available. Example: spec.metadata["extractor:history"] = "Shell command history".

The Registry could then populate itself by looking for these metadata keys in Gem::Specification.filter{|s| s.name.match(/^chronicle-/) }

Attempts to require a plugin (and its dependencies) would only happen at runtime when running a job that uses them and we'd only be using one (or a handful at most), thus reducing the potential for conflict.