manage_token privilege for kibana_system

[albertzaharovits]

Creates the manage_token cluster privilege and adds it to the kibana_system role.
This is required if kibana were to use the token service for its authenticator process.

NB Because kibana_system already has manage_saml this effectively only adds the privilege to create tokens.

CC @epixa

[elasticmachine]

Pinging @elastic/es-security

[tvernum]

👍

[jkakavas]

LGTM