Security updates for logstash system index APIs

[jaymode]

After adding the system index APIs for pipeline management, the security privileges and roles should be updated to account for these APIs.

Specifically:

• Add new NamedClusterPrivilege in ClusterPrivilegeResolver with name manage_logstash_pipelines
• Update logstash_admin in ReservedRolesStore to use the cluster privilege
• Document the new cluster privilege

[elasticmachine]

Pinging @elastic/es-security (:Security/Authorization)