Datasource for Grafana logging platform

- new builtin datasource plugin "Logging" (likely going to be renamed)
- plugin implements no panel ctrls yet, only ships datasource
- new models for logging data as first class citizen (aside from table
  and time_series model)
- Logs as new view for Explore
- JSON view for development

Testable only against existing logish deployment.
Then test with queries like `{job="..."} regexp`.

pkg/plugins/datasource_plugin.go

@@ -17,12 +17,14 @@ import (
 	plugin "github.com/hashicorp/go-plugin"
 )
 
+// DataSourcePlugin contains all metadata about a datasource plugin
 type DataSourcePlugin struct {
 	FrontendPluginBase
 	Annotations  bool              `json:"annotations"`
 	Metrics      bool              `json:"metrics"`
 	Alerting     bool              `json:"alerting"`
 	Explore      bool              `json:"explore"`
+	Logs         bool              `json:"logs"`
 	QueryOptions map[string]bool   `json:"queryOptions,omitempty"`
 	BuiltIn      bool              `json:"builtIn,omitempty"`
 	Mixed        bool              `json:"mixed,omitempty"`