Support for multi-flow decorators.

[valayDave]

PR Stack Merge Order (#1252 --> #1234 --> #1226 --> #1225 --> #1224)

• This change is required to support Airflow Sensors.
• Airflow Sensors will support multiple flow decorators per flow
• This will also be the base PR for the stacked Airflow Foreach + Sensors + Airflow GCP Support PRs

[romain-intel]

A few minor nits.

[romain-intel]

This should be hoisted out of the inner for loop because it makes it look like there are different options for each rd which is not the case (they are shared for a given type of decorator).

[valayDave]

Won't the decorator options be different per decorator instance. For example :

@airflow_sensor_a(myopt1="DEF")
@airflow_sensor_a(myopt1="ABC")
class MyFlow(FlowSpec):

[romain-intel]

Yes, but those are attributes and not options (I got confused too). The attributes are things extracted from the decorator itself and the optiosn are extracted from the command line (so for project, for example, you have name as an attribute and production and branch as options).

To remove this confusion, when we were talking with @savingoyal today, we thought that maybe we could assert that if allow_multiple is true, the options dict is empty. An even better way of doing it is ensuring this at runtime too but it's a bit more complicated (ie: ensuring that it can't be filled in later too). This will prevent some weird things and ppl getting surprised.

[valayDave]

Refactored it. Ready for review.

[romain-intel]

LGTM. Gimme until monday to make my changes and test it out but should be good.