feat(dotfiles): Add an optional coder_app to update dotfiles on-demand

[matifali]

Adds an optional coder_app, that lets a user update dotfiles without a workspace restart.

To test use

module "dotfiles" {
  source        = "git::https://github.com/coder/modules.git//dotfiles?ref=matifali/update-dotfiles"
  agent_id      = coder_agent.dev.id
  manual_update = true
}

This adds a button to update dotfiles on demand.

[cirego]

Thank you!!!

What do you think about calling it refresh or resync instead of update?