fix(agent/agentcontainers/dcspec): generate unmarshalers and add tests (#17330)

This change allows proper unmarshaling of `devcontainer.json` and will
no longer break EnvInfoer or the Web Terminal.

Fixes coder/internal#556

agent/agentcontainers/dcspec/testdata/arrays.json

@@ -0,0 +1,5 @@
+{
+	"image": "test-image",
+	"runArgs": ["--network=host", "--privileged"],
+	"forwardPorts": [8080, "3000:3000"]
+}

agent/agentcontainers/dcspec/testdata/devcontainers-template-starter.json

@@ -0,0 +1,12 @@
+{
+	"image": "mcr.microsoft.com/devcontainers/javascript-node:1-18-bullseye",
+	"features": {
+		"ghcr.io/devcontainers/features/docker-in-docker:2": {}
+	},
+	"customizations": {
+		"vscode": {
+			"extensions": ["mads-hartmann.bash-ide-vscode", "dbaeumer.vscode-eslint"]
+		}
+	},
+	"postCreateCommand": "npm install -g @devcontainers/cli"
+}

agent/agentcontainers/dcspec/testdata/minimal.json

@@ -0,0 +1 @@
+{ "image": "test-image" }