feat(site): connect open_in parameter (#16036)

Second step to resolve [open_in
issue](https://github.com/coder/terraform-provider-coder/issues/297)

This PR improves the way the open_in parameter is forwarded across the
code, changing the last `string` to const everywhere.

Also make sure it is available and forwarded up to the `CreateLink`
component.

docs/reference/api/agents.md

@@ -452,7 +452,7 @@ curl -X GET http://coder-server:8080/api/v2/workspaceagents/{workspaceagent} \
       "hidden": true,
       "icon": "string",
       "id": "497f6eca-6276-4993-bfeb-53cbbbba6f08",
-      "open_in": "string",
+      "open_in": "slim-window",
       "sharing_level": "owner",
       "slug": "string",
       "subdomain": true,