Allow passing string values for ssl query string in PostgreSQL URL

[Exagone313]

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

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.

[Exagone313]

I found later on that you can pass the ssl parameter directly as a keyword argument when initializing the Database class.

But I still believe it could be useful to pass it in the URL.