[tablet, queryrules] Extend query rules to check comments

[setassociative]

Description

This PR makes three changes

1. It introduces an optional us of fsnotify to watch for changes vs the rules file
2. it extends rules evaluation to support comparison vs leading/trailing comments
3. It adds support to specify leading/trailing comments as part of rule creation in rulesctl

Details: Watching the file

We use fsnotify which seems to be the generally accepted approach. There are some gymnastics necessary as watching the specific file is prone to losing track of it in the event that the file is deleted+recreated. To handle this case we watch the directory containing the file and react only when the node we receive events for matches the file we are watching.

Details: Filtering on MarginComments

This was mostly plumbing within the Rule itself + the application callstack as the tabletserver/query_executor::QueryExecutor has already baked margin comments into it during the initial phase of the Execute call.

Evaluation is similar to but not the same as the Query regexp because that is stable across query shapes (otherwise it wouldn't be the same query). For margin comments we need to defer evaluation until after the plan cache which is why Query & comments take different paths.

Test failures

The only listed failure is endtoend/tabletmanager/tablegc which seems unlikely to be related afaict.

Related Issue(s)

n/a

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required.

Deployment Notes

No changes necessary for existing deployments.

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[rafael]

From our discussions, going to merge this to our branch and start playing with it in our environment.