New `template` sharing option for `coder_app`s

Some users want to share `coder_app`s with a specific group or users, rather than all authenticated users on a deployment. We should support this with a new `"template"` [share attribute](https://registry.terraform.io/providers/coder/coder/latest/docs/resources/app#share) for `coder_app`s.

This would open the app to any users who are granted access to the template. While this is a slightly convoluted method of sharing with groups, it best aligns with our current RBAC.

`hcl resource "coder_app" "vim" { agent_id = coder_agent.dev.id slug = "vim" ... share = "template" } `

Related: [https://github.com/coder/coder/issues/13361](https://github.com/coder/coder/issues/13361)