Support Operation class decorators

[dnephin]

There are many use-cases for decorating a client to provide additional functionality. Some common ones that have come up before are:

• wrapping exceptions, either to cast them to another type, log them, or catch them and prevent them from raising
• providing defaults for _request_options (especially headers) to every service call. This is useful for things like tracking ids, authentication headers, force-master-read, etc.
• retry and timeout behaviour. Within a single application it's likely that these will remain pretty consistent, so providing a decorator allows you to re-use the values for all calls

Since our service calls aren't exposed directly from the SwaggerClient there is a bit more work involved to create a decorator (the SwaggerClient only exposes resources, the resources expose operation calls, which are what we want to decorate).

I think we should provide some support for this in bravado (potentially swagger-py as well, they should be reusable for both since the external interface is basically identical).

I think there are two potential options.

• provide a ClientDecorator that knows how to decorate both the resources and operations. This option is less coupled with swagger-py itself, but requires more code.
• have SwaggerClient accept a list of IOperationDecorator objects which conform to some interface and apply the decorators directly to the operation. More tightly coupled with swagger-py, but easier to implement.

I prefer the second option.