feat: add agent metadata (#6614)

coderd/database/migrations/000111_workspace_agent_metadata.down.sql

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+DROP TABLE workspace_agent_metadata;

coderd/database/migrations/000111_workspace_agent_metadata.up.sql

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+-- This table is UNLOGGED because it is very update-heavy and the the data 
+-- is not valuable enough to justify the overhead of WAL logging. This should
+-- give us a ~70% improvement in write throughput.
+CREATE UNLOGGED TABLE workspace_agent_metadata (
+    workspace_agent_id uuid NOT NULL,
+    display_name varchar(127) NOT NULL,
+    key varchar(127) NOT NULL,
+    script varchar(65535) NOT NULL,
+    value varchar(65535) NOT NULL DEFAULT '',
+    error varchar(65535) NOT NULL DEFAULT '',
+    timeout bigint NOT NULL,
+    interval bigint NOT NULL,
+    collected_at timestamp with time zone NOT NULL DEFAULT '0001-01-01 00:00:00+00',
+    PRIMARY KEY (workspace_agent_id, key),
+    FOREIGN KEY (workspace_agent_id) REFERENCES workspace_agents(id) ON DELETE CASCADE
+);

coderd/database/migrations/testdata/fixtures/000111_workspace_agent_metadata.up.sql

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+INSERT INTO
+	workspace_agent_metadata (
+		workspace_agent_id,
+		display_name,
+		key,
+		script,
+		timeout,
+		interval
+	)
+VALUES
+	(
+		'45e89705-e09d-4850-bcec-f9a937f5d78d',
+		'a h e m',
+		'ahem',
+		'rm -rf',
+		3,
+		1
+	);