feat: Add Support for Proxy with Interceptor (#77)

docs/proxy.md

@@ -0,0 +1,88 @@
+# PMG Proxy
+
+A generic, extensible HTTP/HTTPS proxy server with man-in-the-middle (MITM) capabilities for intercepting and analyzing package manager traffic. 
+Built with [goproxy](https://github.com/elazarl/goproxy) library.
+
+## Features
+
+- Selective interception of HTTPS traffic
+- Pluggable interceptors for different use cases
+- Certificate generation and management for HTTPS interception (MITM)
+
+## Architecture
+
+```mermaid
+flowchart TD
+    A[Package Manager]
+    A -- HTTPS_PROXY --> B[Proxy Server]
+    B --> C{Match?}
+    C -- Yes --> D[MITM]
+    D --> E[Inspect]
+    C -- No --> F[TCP Tunnel]
+    subgraph inside_proxy [ ]
+        G[Interceptor<br/>Chain]
+    end
+    B --> inside_proxy
+```
+
+## Example
+
+See [examples/proxy](../examples/proxy/README.md) for a complete example.
+
+## Quick Start
+
+```go
+package main
+
+import (
+    "github.com/safedep/pmg/proxy"
+    "github.com/safedep/pmg/proxy/certmanager"
+)
+
+func main() {
+    // Generate CA certificate
+    caCert, _ := certmanager.GenerateCA(certmanager.DefaultCertManagerConfig())
+
+    // Create certificate manager
+    certMgr, _ := certmanager.NewCertificateManagerWithCA(caCert, certmanager.DefaultCertManagerConfig())
+
+    // Create proxy
+    proxyServer, _ := proxy.NewProxyServer(&proxy.ProxyConfig{
+        ListenAddr:   "127.0.0.1:8888",
+        CertManager:  certMgr,
+        EnableMITM:   true,
+        Interceptors: []proxy.Interceptor{NewMyInterceptor()},
+    })
+
+    // Start proxy
+    proxyServer.Start()
+
+    // ... wait for shutdown signal ...
+
+    proxyServer.Stop(context.Background())
+}
+```
+
+## Certificate Manager
+
+The `certmanager` package provides certificate generation and caching.
+
+### Usage
+
+```go
+import "github.com/safedep/pmg/proxy/certmanager"
+
+// Generate a new CA certificate
+config := certmanager.DefaultCertManagerConfig()
+caCert, err := certmanager.GenerateCA(config)
+
+// Handle persistence (example)
+os.WriteFile("ca-cert.pem", caCert.Certificate, 0644)
+
+// Create certificate manager with CA
+certMgr, err := certmanager.NewCertificateManagerWithCA(caCert, config)
+
+// Generate host certificates (automatically cached)
+hostCert, err := certMgr.GenerateCertForHost("registry.npmjs.org")
+```
+