special case time and duration conversions

Consider adding special case handling for `time.Duration` and `time.Time` conversions to/from various protobuf related equivalent types so there is no need for helper conversion functions.

Start by locating the Go code-generation entry points that handle conversions between API and core types. Trace how time.Duration and time.Time are currently converted, then identify the protobuf-equivalent types involved. Done means generated conversions cover these cases without requiring helper conversion functions; the issue provides no named files or tests.

Written by the indexing model from the issue text.