V2.x.x Add a generate attribute to meta-rules

[frack113]

From meta-rules V2.0.0 discussion

• All rules in a file, basic event rules as well as correlations, might contain an additional attribute called "generate".
• If it is set to true, the rule will generate a query, even if it is referred to by other correlations. Otherwise by default no "standalone" query would be generated for this rule.

Usually if a Sigma rule is referenced by a correlation rule the query for the rule itself is not generated anymore.
This attribute overrides the behavior.
The idea is that two rules are created:

• one for the singular event with informational severity.
• the correlation rule that takes appearance of multiple events into account with a higher severity.