Performer custom fields (#5487)

* Backend changes
* Show custom field values
* Add custom fields table input
* Add custom field filtering
* Add unit tests
* Include custom fields in import/export
* Anonymise performer custom fields
* Move json.Number handler functions to api
* Handle json.Number conversion in api

pkg/models/custom_fields.go

@@ -0,0 +1,17 @@
+package models
+
+import "context"
+
+type CustomFieldMap map[string]interface{}
+
+type CustomFieldsInput struct {
+	// If populated, the entire custom fields map will be replaced with this value
+	Full map[string]interface{} `json:"full"`
+	// If populated, only the keys in this map will be updated
+	Partial map[string]interface{} `json:"partial"`
+}
+
+type CustomFieldsReader interface {
+	GetCustomFields(ctx context.Context, id int) (map[string]interface{}, error)
+	GetCustomFieldsBulk(ctx context.Context, ids []int) ([]CustomFieldMap, error)
+}