(feat, fastapi): introduce endpoint specific async handlers in fastapi

[dsinghvi]

Feature: The FastAPI async_handlers configuration now accepts a list of string or a boolean so that users
can turn on async handlers for specific endpoints. For example, to turn on async handlers for a users.create
endpoint, use the configuration below:

- name: fernapi/fern-fastapi-server
  version: 1.1.1
  config:
    async_handlers:
      - users.create

[armandobelardo]

Nit: 1.2.0? Just cause it's a feature not a fix

[armandobelardo]

LGTM!
The endpoint list isn't the easiest to maintain as your endpoints grow and change, but not sure how else we'd do it without modifying the fern def which I know we don't want do do here