feat: configure support for generic integrations

Co-authored-by: Gage Orsburn <gageorsburn@live.com>

app/integration/index.ts

@@ -0,0 +1,57 @@
+import docker from './docker'
+import proc from './proc'
+
+export interface Integration {
+	preflight: () => Promise<boolean>
+	sighup?: () => Promise<void>
+	restart?: () => Promise<void>
+}
+
+// Because we previously supported the Docker integration by
+// checking for the HEADSCALE_CONTAINER variable, we need to
+// check for it here as well.
+//
+// This ensures that when people upgrade from older versions
+// of Headplane, they don't explicitly need to define the new
+// HEADSCALE_INTEGRATION variable that is needed to configure
+// an integration.
+export async function checkIntegration() {
+	let integration = process.env.HEADSCALE_INTEGRATION
+		?.trim()
+		.toLowerCase()
+
+	// Old HEADSCALE_CONTAINER variable upgrade path
+	if (!integration && process.env.HEADSCALE_CONTAINER) {
+		integration = 'docker'
+	}
+
+	if (!integration) {
+		console.log('Running Headplane without any integrations')
+		return
+	}
+
+	let module: Integration | undefined
+	try {
+		module = getIntegration(integration)
+		await module.preflight()
+	} catch (error) {
+		console.error('Failed to load integration', error)
+		return
+	}
+
+	return module
+}
+
+function getIntegration(name: string) {
+	switch (name) {
+		case 'docker': {
+			return docker
+		}
+		case 'proc': {
+			return proc
+		}
+		default: {
+			throw new Error(`Unknown integration: ${name}`)
+		}
+	}
+}