Add support for deserialization Duration in HOCON format

[alexmihailov]

Currently kotlin.time.Duration by default supports deserialization from ISO-8601-2 format. But HOCON uses a different format.
Made changes so that when deserializing kotlin.time.Duration from HOCON, by default use the format specified in the documentation.

[sandwwraith]

Overall the PR is good, but I have several thoughts:

• Hocon format also supports encoding to config (Hocon.encodeToConfig). Do you plan to support Duration in encoding, too? If so, how will it be encoded? Milliseconds only?
• Documentation should be updated to reflect this change. Since we do not have a separate HOCON entry in https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/serialization-guide.md, KDoc update should do the job.
• Does it make sense to add a flag to restore old behavior? If one wants now to serialize Duration in ISO8601, they would have to resort to cumbersome workarounds like making a custom serializer that delegates to duration serializer but uses a different descriptor. However, I'm not well aware of hocon usages and if no one does ISO Duration serialization, I'm fine with not having flag.

[ihostage]

Hi, Leonid @sandwwraith!

• Hocon format also supports encoding to config (Hocon.encodeToConfig). Do you plan to support Duration in encoding, too? If so, how will it be encoded? Milliseconds only?

Yes, we discussed this with Alex @alexmihailov and decided to implement this in separate PR later. You are right, it will be something like we implemented this case In Config4K
https://github.com/config4k/config4k/blob/20294740a3735851e20287bdf38874b2bc1f6b63/src/main/kotlin/io/github/config4k/Extension.kt#L97
In this case, it's hard to come up with something striking better 🤷‍♂️

• Does it make sense to add a flag to restore old behavior? If one wants now to serialize Duration in ISO8601, they would have to resort to cumbersome workarounds like making a custom serializer that delegates to duration serializer but uses a different descriptor. However, I'm not well aware of hocon usages and if no one does ISO Duration serialization, I'm fine with not having flag.

I think it's not worth it. Typesafe Config as HOCON reference implementation doesn't support ISO8601 and I've never seen that someone use this format for Duration 😄 I think we will be able to add a flag by request in the future if I'm wrong and it's really needed for someone.

[sandwwraith]

I think it's not worth it. Typesafe Config as HOCON reference implementation doesn't support ISO8601 and I've never seen that someone use this format for Duration

OK, no problem

[sandwwraith]

Apart from minor documentation rewording, I do not have suggestions. However, I kindly ask you to submit encoding PR before the next release (which should happen in about a month), so the format continues to look consistent.

[alexmihailov]

Apart from minor documentation rewording, I do not have suggestions. However, I kindly ask you to submit encoding PR before the next release (which should happen in about a month), so the format continues to look consistent.

For encoding, I already have a solution, I will submit a PR soon

[sandwwraith]

Thank you, great job!