Allow serving custom RuleSet by plugins

[wata727]

This PR changes the tflint.Ruleset struct to the interface. This allows each plugin to serve a custom ruleset.

The previous behavior can be reproduced by using the tflint.BuiltinRuleSet. If you do not need plugin-specific processing, please use tflint.BuiltinRuleSet directly.

Custom rulesets can be used by declared a RuleSet with a tflint.BuiltinRuleSet embedded in the plugin and serving it. By overwriting the following method, you can insert the process specific to the plugin.

• ApplyConfig
  • The argument tflint.Config contains the hcl.Body defined in the "plugin" block. You can handle plugin-specific configurations by decoding this with a custom ruleset.
• Check
  • You can generate custom configurations built with ApplyConfig, custom runners that wrap provider API clients, etc., and apply them to plugin rules.