Add `default:` for `connection_info` attributes on transports

Use Case

Optional attributes for transport connection_info often have default values. Currently this requires calling out the default value in the desc string and defining a fallback calculation in code, leading to duplication at best and inconsistency at worst.

Describe the Solution You Would Like

Define a default: key equivalent to the attributes in types.

Start by tracing how transport connection_info attributes are defined and how type attributes currently handle defaults. Compare the existing description and fallback behavior, then determine the tests covering connection_info defaults. Done means transports can declare a default value through a default: key without duplicating it in descriptions or fallback calculations.