text: Title: Allow passing string values for ssl query string in PostgreSQL URL
  Body: When using the PostgreSQL backend, you can pass the `ssl` key in the query string, with the values `true` or `false`. These are converted to boolean and passed as arguments to [asyncpg.create_pool](https://magicstack.github.io/asyncpg/current/api/index.html#asyncpg.pool.create_pool)
  
  The asyncpg library accepts other values than only `True` and `False`, which can be used to choose how certificate validation is done (or not done).
  
  ~~For the record, when setting `ssl=true`, the ssl mode used is `prefer`, which will fallback to plain if SSL connection fails, so it is not a secure default.~~ (Edit: This is not true, certificate is checked with `ssl=true`, the documentation is not clear on that topic).
  
  I'm going to send a PR that permits to send string values, but it will not change the default settings.