Backend Plugins: Provide proper plugin config to plugins (#21985)

Properly provides plugin configs to backend plugins.
Uses v0.16.0 of grafana-plugin-sdk-go-

Ref #21512
Ref #19667

pkg/models/plugin_setting_cache.go

@@ -0,0 +1,37 @@
+package models
+
+var pluginSettingDecryptionCache = secureJSONDecryptionCache{
+	cache: make(map[int64]cachedDecryptedJSON),
+}
+
+// DecryptedValues returns cached decrypted values from secureJsonData.
+func (ps *PluginSetting) DecryptedValues() map[string]string {
+	pluginSettingDecryptionCache.Lock()
+	defer pluginSettingDecryptionCache.Unlock()
+
+	if item, present := pluginSettingDecryptionCache.cache[ps.Id]; present && ps.Updated.Equal(item.updated) {
+		return item.json
+	}
+
+	json := ps.SecureJsonData.Decrypt()
+	pluginSettingDecryptionCache.cache[ps.Id] = cachedDecryptedJSON{
+		updated: ps.Updated,
+		json:    json,
+	}
+
+	return json
+}
+
+// DecryptedValue returns cached decrypted value from cached secureJsonData.
+func (ps *PluginSetting) DecryptedValue(key string) (string, bool) {
+	value, exists := ps.DecryptedValues()[key]
+	return value, exists
+}
+
+// ClearPluginSettingDecryptionCache clears the datasource decryption cache.
+func ClearPluginSettingDecryptionCache() {
+	pluginSettingDecryptionCache.Lock()
+	defer pluginSettingDecryptionCache.Unlock()
+
+	pluginSettingDecryptionCache.cache = make(map[int64]cachedDecryptedJSON)
+}