feat(plugins): add plugin config options

Add configuration keys:
- plugins.enabled (default: false)
- plugins.dir (default: <rootpath>/plugins)
- plugins.loader (default: native, validated at startup)

Register yaegi loader via blank import in initialize package.

config-raw.json

@@ -1049,6 +1049,11 @@
                     "key": "dir",
                     "default_value": "<rootpath>plugins",
                     "comment": "The directory where plugins are stored."
+                },
+                {
+                    "key": "loader",
+                    "default_value": "native",
+                    "comment": "The plugin loader to use. \"yaegi\" loads plugins from Go source files (directories of .go files). \"native\" (deprecated) loads compiled Go plugin shared libraries (.so files)."
                 }
             ]
         }

pkg/config/config.go

@@ -230,6 +230,7 @@ const (
 
 	PluginsEnabled Key = `plugins.enabled`
 	PluginsDir     Key = `plugins.dir`
+	PluginsLoader  Key = `plugins.loader`
 )
 
 var maxFileSizeInBytes uint64
@@ -484,6 +485,7 @@ func InitDefaultConfig() {
 	// Plugins
 	PluginsEnabled.setDefault(false)
 	PluginsDir.setDefault(ResolvePath("plugins"))
+	PluginsLoader.setDefault("native")
 
 	// Migrate deprecated webhook config keys to outgoingrequests.*
 	// This allows removing the old keys in a single place later.
@@ -657,6 +659,10 @@ func InitConfig() {
 		RateLimitStore.Set(KeyvalueType.GetString())
 	}
 
+	if loader := PluginsLoader.GetString(); loader != "yaegi" && loader != "native" {
+		log.Fatalf("Invalid value for plugins.loader: %q (must be \"yaegi\" or \"native\")", loader)
+	}
+
 	if CorsEnable.GetBool() && ServicePublicURL.GetString() == "" {
 		log.Fatalf("service.publicurl is required when cors.enable is true")
 	}

pkg/initialize/init.go

@@ -34,6 +34,7 @@ import (
 	"code.vikunja.io/api/pkg/modules/keyvalue"
 	migrationHandler "code.vikunja.io/api/pkg/modules/migration/handler"
 	"code.vikunja.io/api/pkg/plugins"
+	_ "code.vikunja.io/api/pkg/plugins/yaegi" // register yaegi plugin loader
 	"code.vikunja.io/api/pkg/red"
 	"code.vikunja.io/api/pkg/user"
 )