Native multi-file YAML config support like Nginx `include` directive

## Feature Request Summary

Add native support for splitting Envoy bootstrap configuration across multiple YAML files, similar to Nginx's `include` directive and `/etc/nginx/conf.d/*.conf` directory scanning.

## Motivation

Unlike Nginx, which seamlessly loads multiple config files from a directory into a unified config tree, Envoy requires a single bootstrap YAML file. Current workarounds like `!ignore` YAML fragments with anchors/aliases work but have limitations:

- Still requires a "main" bootstrap file
- Manual anchor management across files
- No automatic directory scanning
- Full reload needed for fragment changes
- Not as intuitive for teams familiar with Nginx/HAProxy

This is especially painful for complex deployments (Kubernetes sidecars, Docker Compose with multiple services) where configs grow large and need modularization.