Add slow SQL query warning

[earl-warren]

• Databases are one of the most important parts of Forgejo, every interaction uses the database in one way or another. Therefore, it is important to maintain the database and recognize when the server is not doing well with the database. There already is the option to log every SQL query along with its execution time, but monitoring becomes impractical for larger instances and takes up unnecessary storage in the logs.
• Add a QoL enhancement that allows instance administrators to specify a threshold value beyond which query execution time is logged as a warning in the xorm logger. The default value is a conservative five seconds to avoid this becoming a source of spam in the logs.
• The use case for this patch is that with an instance the size of Codeberg, monitoring SQL logs is not very fruitful and most of them are uninteresting. Recently, in the context of persistent deadlock issues (https://codeberg.org/forgejo/forgejo/issues/220), I have noticed that certain queries hold locks on tables like comment and issue for several seconds. This patch helps to identify which queries these are and when they happen.
• Added unit test.

(cherry picked from commit 9cf501f1af4cd870221cef6af489618785b71186)

[JakobDev]

The new Setting needs to be documented

[wxiaoguang]

More thoughts about this feature:

1. Is it useful? Somewhat, but not really. DBA should already have the ability to find the slow queries from database log.
2. Is it complete? I think no, because there are also many performance problems are caused "plenty SQLs in a loop", say, even if a single SQL is fast, if it is executed in a loop, the whole request will slow down. But this PR isn't able to catch this case.

So I think it is fine to merge, although it is far from ideal.

ps: my previous about the "tests", I never meant that it doesn't need tests, but actually that test code was wrong, so I think the tests should be rewritten correctly.

[6543]

i think it's a usefull tool ...
... yes it does not cover the whole picture, but if it catches at lest some, its better than nothing