scylla-5.2.4+criteo1

auth: add support for authc/authz via external api (#10)

Squash a serie of commits related to authentication found on
v5.0.1-criteo branch:
* 34fe8d4ef11962a5f56772641193886bbbf07c9e Add attributes support
* bf5cf176fb8deaee04e70f9c7f24eb18e4ccd496 Implement List Roles
* fe21809bc5d7288545ea9aa21ef05a0d29f07336 Add alter user implementation to the rest role manager
* 69fd387e7103245cce998216af706b70c9c242b3 Update ssl cert
* 6a32079f6d6e3e2b7809b021f32c433b67e76c4d Add authorization support with rest_auth
* 2f92f8b8d182aa23992c9ac6d7de5775de16498a deprecate internal_distributed_timeout_config
* a39e509b4fa3df5da84300d75d444aa4c92e3f8c Add rest_authenticator to manage authentication with a rest endpoint validating credentials

Few changes introduced compared to 5.0.1 version:
* fixed runtime assertion related to pending flush when scylla
fails to communicate with rest auth api
* picojson replaced by rapidjson (used by scylla)
* rest_http_client replaced by seastar::http::experimental::connection
* unit tests fixed and enriched
* formatting aligned with rest of scylla code base
* tools/rest_authenticator_server updated to match actual
implementation (usage of TLS and of GET http verb instead
of POST)