Prebuild TTL Configuration Support

[ssncferreira]

Description

Introduce support for a TTL (Time-To-Live) configuration for prebuilt workspaces, enabling automatic invalidation and replacement of unclaimed prebuilds after a specified duration.

When defining a prebuild in Terraform, users should be able to set the maximum lifetime of a prebuild using the invalidate_after_secs field under the cache_invalidation block. After this duration has passed, the prebuild should be considered stale and be replaced automatically.

prebuilds = {
  instances = 2
  cache_invalidation = {
    invalidate_after_secs = 86400 // 1 day
  }
}

This feature will help prevent stale workspaces due to outdated codebases or infrastructure changes (e.g., new VM images).