mcp: make request headers available to tools (#333)

Add HTTP request headers to RequestExtra, so tools and other
user-defined handlers can access them.

Fixes #331.

mcp/shared.go

@@ -16,6 +16,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"net/http"
 	"reflect"
 	"slices"
 	"strings"
@@ -400,6 +401,7 @@ type ServerRequest[P Params] struct {
 // the transport layer.
 type RequestExtra struct {
 	TokenInfo *auth.TokenInfo // bearer token info (e.g. from OAuth) if any
+	Header    http.Header     // header from HTTP request, if any
 }
 
 func (*ClientRequest[P]) isRequest() {}