Offer humantime_serde_option to humantime_serde upstream

!391 (merged) introduces a new module humantime_serde_option. We should offer this to huamntime_serde upstream. !391 (comment 2785550)

assigned to @Diziet

Upstream issue https://github.com/jean-airoldie/humantime-serde/issues/5

Upstream MR https://github.com/jean-airoldie/humantime-serde/pull/6

added Doing label

mentioned in merge request !406 (merged)

mentioned in commit ff489ffd

closed with merge request !406 (merged)