Standardize `transaction_style` option

[szokeasaurusrex]

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, Flask, and Falcon 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.