* Added global http client
* Added Do func for global http client
* Changed the code to use the global http client
* Fix http client in volume uploader
* Fixed pkg name
* Fixed http util funcs
* Fixed http client for bench_filer_upload
* Fixed http client for stress_filer_upload
* Fixed http client for filer_server_handlers_proxy
* Fixed http client for command_fs_merge_volumes
* Fixed http client for command_fs_merge_volumes and command_volume_fsck
* Fixed http client for s3api_server
* Added init global client for main funcs
* Rename global_client to client
* Changed:
- fixed NewHttpClient;
- added CheckIsHttpsClientEnabled func
- updated security.toml in scaffold
* Reduce the visibility of some functions in the util/http/client pkg
* Added the loadSecurityConfig function
* Use util.LoadSecurityConfiguration() in NewHttpClient func
