[+] add stat_plans metric (#14)

* First working version of the `stats_plans` metric.

This metric uses the new `pg_stat_plans` extension to
provide insights about query planning and performance of
different plans for a specific query.

* Refactor the `stat_plans` metric query.

Refactor the metric query to return all plans for
top resource intensive queries which is more useful in deubgging.

The query retrieves the 'top resource intensive' queries via
copying parts of the `stat_statements` metric query.

* Remove returning `avg_exec_time` from query.

* Add `Stat Plans Top` dashboard.

The plan mimics `Stat Statements Top` but provides
insights for top plans for a given query id.

* add `/* pgwatch_generated */` to stat_plans query.

* Join `Total runtime`, `Avg runtime`, and `Calls` in a single sortable panel.

* Move panel to `single-query-details` dashboard

* Limit collected plan chars to 15000

Author: 0xgouda