Add config for setting search_min_length capability

This adds the "search_min_length" setting to the frontend service which
allows to set the search_min_length capabilty which is e.g. used by
web.

Partial: #547

services/frontend/pkg/config/defaults/defaultconfig.go

@@ -39,6 +39,7 @@ func DefaultConfig() *config.Config {
 		UploadHTTPMethodOverride: "",
 		DefaultUploadProtocol:    "tus",
 		EnableResharing:          true,
+		SearchMinLength:          3,
 		Checksums: config.Checksums{
 			SupportedTypes:      []string{"sha1", "md5", "adler32"},
 			PreferredUploadType: "",