Make query time accessible through Jinja template syntax

[addisonklinke]

First off thanks for this amazing project! It automates so much of the front-end boilerplate for dashboards

I'm new to BQL and can tell that my queries (both number and complexity) are rather inefficient based on the load time for a dashboard page. After all the plumbing works, I'd like to go back and optimize this. It'd be really convenient if fava-dashboards recorded the total query time and made it accessible through Jinja like {{queries[i].time}}. This way I could render it on the page itself as feedback

[andreasgerstmayr]

I think the query duration could be exposed in the logs (debug log level), and possible for queries taking longer than some threshold a different log level (warn).

I won't have time to work on this, but I'm happy to help/review a PR.