Add `sql.TypedLiteral`

[dvarrazzo]

In order to make client-side queries more similar to the server-side ones, we can introduce a sql.TypedLiteral object, which, on as_string(), would:

• use the transformer to find the object oid
• use the TypesRegistry to find the name of the type
• add a cast to the object representation (e.g. '2021-01-01'::date).

Note that this might require to create many more Transformer. A worthwile optimisation (which could be useful for any recursive type) could be to not create a transformer when a transformer is passed as a context, but use always the same instance.

[dvarrazzo]

Rather than adding a new type, exploring the possibility to just change the values returned by sql.Literal - see branch typed-literal.