Add ActiveSupport signatures for time methods

[olivier-thatch]

Type of Change

• Add RBI for a new gem
• Modify RBI for an existing gem
• Other:

Changes

• Gem name: activesupport
• Gem version: 8.0.1 (but should be compatible with previous versions)
• Gem source: https://github.com/rails/rails/tree/main/activesupport
• Gem API doc: https://api.rubyonrails.org/classes/ActiveSupport.html
• Tapioca version: Tapioca v0.16.9
• Sorbet version: Sorbet typechecker 0.5.11813 git e125bae4c662972686885a21e05d0df7b27d8640 debug_symbols=true clean=1

This PR adds a bunch of signatures for time-related ActiveSupport methods. I wrote these signatures almost a year ago and we've been using them in our codebase without issues, so I'm fairly confident that they are correct.