Add standard config support and auth token for swh-scanner

• impl a full SWH config scheme, which loads config like the following

(descending priority):

• • CLI switch for config path
  • CLI envvar for component config path (click auto_envvar feature)
  • CLI envvar for global config path
  • default global config path under ~/.config/swh/global.yml
  • merge loaded config with default specific config mapping
• for now the default config path is hardcoded but will move to swh.core.config later
• url and token config is namespaced with web-api to start having a

consistent hierarchical config model accross components

• added simple HTTP auth token support for requests to the SWH API,

without adding yet another dependency for a small middleware

Close T2572