New database backend, MongoDB

I know that implementing a Non-SQL database can seem daunting, but this is one of the most popular database.

If that support is added to Diesel-rs it will probably end up in vaultwarden at some time, but until then i do not see it happening.

Diesel-rs will not support non-sql, from developer statement

Just out of interest, if I made a pull request with a mongoDB implementation, for example in the src/db/non_sql directory, and additional conditions for selecting a DB, would it be likely to be accepted?

It seems unlikely that there would be a clean way to do this, so probably not. Even if there were, the burden of future maintenance for an entirely separate database backend would be unappealing. Of course, you’re welcome to maintain and share your own fork if MongoDB support is that important to you.