feat: implement expiration policy logic for prebuilds (#17996)

## Summary 

This PR introduces support for expiration policies in prebuilds. The TTL
(time-to-live) is retrieved from the Terraform configuration
([terraform-provider-coder
PR](https://github.com/coder/terraform-provider-coder/pull/404)):
```
prebuilds = {
	  instances = 2
	  expiration_policy {
		  ttl = 86400
	  }
  }
```
**Note**: Since there is no need for precise TTL enforcement down to the
second, in this implementation expired prebuilds are handled in a single
reconciliation cycle: they are deleted, and new instances are created
only if needed to match the desired count.

## Changes

* The outcome of a reconciliation cycle is now expressed as a slice of
reconciliation actions, instead of a single aggregated action.
* Adjusted reconciliation logic to delete expired prebuilds and
guarantee that the number of desired instances is correct.
* Updated relevant data structures and methods to support expiration
policies parameters.
* Added documentation to `Prebuilt workspaces` page
* Update `terraform-provider-coder` to version 2.5.0:
https://github.com/coder/terraform-provider-coder/releases/tag/v2.5.0

Depends on: https://github.com/coder/terraform-provider-coder/pull/404
Fixes: https://github.com/coder/coder/issues/17916

coderd/database/queries/prebuilds.sql

@@ -35,6 +35,7 @@ SELECT
 		tvp.id,
 		tvp.name,
 		tvp.desired_instances       AS desired_instances,
+		tvp.invalidate_after_secs 	AS ttl,
 		tvp.prebuild_status,
 		t.deleted,
 		t.deprecated != ''          AS deprecated