feat: Update Terraform provider to support "dir" in "coder_agent" (#1219)

This allows users to specify a starting directory for shell sessions.

coderd/database/migrations/000009_agent_directory.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE ONLY workspace_agents
+	DROP COLUMN IF EXISTS directory;

coderd/database/migrations/000009_agent_directory.up.sql

@@ -0,0 +1,3 @@
+ALTER TABLE ONLY workspace_agents
+    -- UNIX paths are a maximum length of 4096.
+    ADD COLUMN IF NOT EXISTS directory varchar(4096) DEFAULT '' NOT NULL;