Standardize `transaction_style` option

Many server framework integrations take a `transaction_style` parameter. This is a string which specifies how the transaction name is generated. Although many frameworks have similar options, they are named differently; the different possible values for `transaction_style` are not standardized across different integrations (e.g. [AIOHTTP](http://localhost:3000/platforms/python/integrations/aiohttp/#transaction_style), [Flask](http://localhost:3000/platforms/python/integrations/flask/#options), and [Falcon](http://localhost:3000/platforms/python/integrations/falcon/#options) each define very different options).

We should settle on standardized values for this parameter to use across all integrations, perhaps by using an enum. Maybe we can reuse the `TRANSACTION_SOURCE` constants for this purpose.