plugin: provide a strategy to extend tableau ability by third-party

[wenchy]

Caddy (The Ultimate Server) is easy to extend because of its modular architecture. Most kinds of Caddy extensions (or plugins) are known as modules if they extend or plug into Caddy's configuration structure. To be clear, Caddy modules are distinct from Go modules (but they are also Go modules).

See Extending Caddy and https://pkg.go.dev/plugin