feat: add OpenIn option to coder_app (#15743)

This PR is the coder/coder part of [the open_in parameter
issue](https://github.com/coder/terraform-provider-coder/issues/297)
aiming to add a new optional parameter to choose how to open modules.

This PR is heavily linked [to this
PR](https://github.com/coder/terraform-provider-coder/pull/321).

ℹ️ For now, some integrations tests can not be pushed as it requires a
release on the terraform-provider repo.

coderd/database/migrations/000282_workspace_app_add_open_in.down.sql

@@ -0,0 +1,3 @@
+ALTER TABLE workspace_apps DROP COLUMN open_in;
+
+DROP TYPE workspace_app_open_in;

coderd/database/migrations/000282_workspace_app_add_open_in.up.sql

@@ -0,0 +1,3 @@
+CREATE TYPE workspace_app_open_in AS ENUM ('tab', 'window', 'slim-window');
+
+ALTER TABLE workspace_apps ADD COLUMN open_in workspace_app_open_in NOT NULL DEFAULT 'slim-window'::workspace_app_open_in;