Add OptTimedelta datastore option

[smcintyre-r7]

Summary

PR #20755 is enabling us to adjust the clock for Kerberos authentication to account for any skew that may be present. The functionality of allowing the user to specify a timedelta (positive or negative) seems like it could be useful elsewhere. Some examples off the top of my head could be, "I want this certificate to be valid for this long", "I want to run this attack for this long", or "I want this object to appear as though it was made this long ago", etc.

There should be an easy option to specify that the timedelta should be positive or negative. Specifying it must be positive for validation will probably see more use because that'd be analogous with a timespan.

Basic example

Motivation

This seems like useful functionality that we could leverage elsewhere.

[ptownes-r7]

Looks like we have ActiveSupport in the gemspec already, so for safe parsing we could maybe do something like:

require 'active_support/core_ext/numeric/time' unless Numeric.method_defined?(:days)

UNITS = {
  "s" => :seconds,
  "m" => :minutes,
  "h" => :hours,
  "d" => :days,
  "w" => :weeks,
}.freeze

def parse_duration(str)
  match = /\A([+-]?\d+)([smhdw])?\z/.match(str)
  raise ArgumentError, "Invalid duration: #{str}" unless match

  value, unit = match.captures
  value = value.to_i

  value.public_send(UNITS.fetch(unit, :seconds)).to_i
end