Core: Create DURATION settings type

[clemyan]

What's the problem this PR addresses?

There are a number of configurations that represent a duration of time (e.g. httpTimeout). Currently those use the NUMBER settings type with a predefined unit (e.g. milliseconds for httpTimeout). But:

1. Each setting uses a different unit so users need to context switch to the docs to read/write those config
2. Because YAML doesn't support arithmetic expressions, unit conversions need to be done beforehand in order to properly set those settings.

How did you fix it?

Create a new DURATION settings type that allows specifying the duration as a string in the following format:

1. An integer or decimal value
2. Optionally a unit among ms, s, m, h, d, w

For backward compatibility, a DURATION is parsed to a number using the existing predefined unit for that setting. That unit is also used if one is not specified.

I have only implemented units that the exactly convertible to each other* to avoid something like having to convert 1 month to an exact number of days.

_{(* not entirely true since 1 day is not always 24 hours)}

In the future, maybe we can use a proper duration data structure to do proper date calculations. (So that, for example, setting a DURATION to 1 month actually means "1 calendar month" instead of an approximate number of seconds)

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.