Should we have universal connector context variables?

[gmmorris]

Relates to #112079

When testing connectors there are no context variables available.
Adding some universal context variables (that are always available) would enable customers to test how they interact with context variables prior to setting up a rule.

This is not an alternative to the ability to test them on a rule (which would mean we can add context-dependent dummy variables) but it could help customers when testing the connectors from Stack Management.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[pmuellr]

I did a quick look when looking at the support case for this. Seems like the only existing top-level mustache variables that would make sense would be date and kibanaBaseUrl. The rest are alerting-related, so seem a bit out of scope. Turns out for the support case, the user was looking to use the date variable ...