feat: add backend logic for determining tasks tab visibility (#18401)

This PR implements the backend logic for determining if the Tasks tab
should be visible in the web UI as described in [the
RFC](https://www.notion.so/coderhq/Coder-Tasks-207d579be5928053ab68c8d9a4b59eaa?source=copy_link#210d579be5928013ab5acbe69a2f548b).

The frontend component will be added in a follow-up PR once the entire
Tasks backend is implemented so as not to break the dogfood environment
until then.

coderd/apidoc/docs.go

@@ -12483,6 +12483,9 @@ const docTemplate = `{
                 "healthcheck": {
                     "$ref": "#/definitions/codersdk.HealthcheckConfig"
                 },
+                "hide_ai_tasks": {
+                    "type": "boolean"
+                },
                 "http_address": {
                     "description": "HTTPAddress is a string because it may be set to zero to disable.",
                     "type": "string"