Support duration parsing that omits units

Try to provide some flexibility like Spring Boot does, allowing for units suffix to be omitted and with a declarative default unit.

https://share.google/aimode/zyavuyQourNlebNnT

I'm not sure if looking at another field tag or indicator of it can be done from the final parse operation.

Would help with https://github.com/itzg/mc-router/issues/551

The issue names no files or tests; start by tracing duration parsing and the final parse operation in this Go repository, including how field metadata is available. Confirm the intended declarative default-unit indicator, then verify that duration values work both with an explicit suffix and without one using that default.

Written by the indexing model from the issue text.