[Actions] Preview mustache variable contents

[jonathan-buttner]

Various external integrations have limits on the amount of data that can be provided for certain fields. It would be helpful for customers to get a warning or insight into whether the result of a mustache variable will exceed the limit on a field. For example, Opsgenie has a limit of 130 characters on the message field. If the limit is exceeded the action will fail.

Related issues: #49411

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[pmuellr]

@jonathan-buttner, I think we'll need some suggestions for how we would handle this. While the web UX can render markdown locally, to measure it's size, we won't have all the variables available, so we really can't render it.

We've had some thoughts about "sample context data" a rule type would make available - it would probably have to be several flavors for some of the rules (various types of grouping, etc). Even if we had that, it would be a guess. Here's one of the oldest alerting issues still open, referencing that: [Actions] Ability to test executing an action from UI #49372

[mikecote]

Closing in favour of https://github.com/elastic/response-ops-team/issues/265