Deserialize time as Duration

[lukellmann]

The Duration type is Kotlin's way to express time amounts.
However, it is not serializable by default and time is serialized in different units across Discord's API.

To make Duration usable in serializable classes, this PR introduces a serializer for every DurationUnit that (de-)serializes Durations as an integer number of the respective DurationUnit.

It also changes the types of rate_limt_per_user, max_age and afk_timeout to Duration, using the new serializers. ArchiveDuration now also uses Duration as the backing value.

[lukellmann]

In places where I had to annotate type parameters like

val rateLimitPerUser: Optional<@Serializable(with = DurationInSecondsSerializer::class) Duration> = Optional.Missing()

I've added import aliases to make it shorter and more readable

val rateLimitPerUser: Optional<@Serializable(InSeconds::class) Duration> = Optional.Missing()

Edit:
To make it even more readable, I've added some typealiases:

typealias DurationInSeconds = @Serializable(with = DurationInSecondsSerializer::class) Duration

val rateLimitPerUser: Optional<DurationInSeconds> = Optional.Missing()

[lukellmann]

I haven't changed the type to Duration in cases where time is expressed as an integer number of days yet.

I suggest to only do this in places where the time unit is not already part of the property name (like DiscordIntegration.expireGracePeriod) and don't change it where the unit is mentioned in the property name (like GuildBanCreateRequest.deleteMessageDays).

Edit:
I implemented my suggestion.