Add file upload support with body_FormData and file_Base64 functions

- Add FileCache for caching local files and remote URLs in memory
- Update body_FormData to accept variadic key-value pairs with file support
  - Use @ prefix for file paths (local or HTTP/HTTPS URLs)
  - Use @@ to escape literal @ values
- Add file_Base64 function for Base64 encoding files
- Update documentation with new syntax and examples

internal/sarin/request.go

@@ -34,11 +34,12 @@ func NewRequestGenerator(
 	cookies types.Cookies,
 	bodies []string,
 	values []string,
+	fileCache *FileCache,
 ) (RequestGenerator, bool) {
 	randSource := NewDefaultRandSource()
 	//nolint:gosec // G404: Using non-cryptographic rand for load testing, not security
 	localRand := rand.New(randSource)
-	templateFuncMap := NewDefaultTemplateFuncMap(randSource)
+	templateFuncMap := NewDefaultTemplateFuncMap(randSource, fileCache)
 
 	pathGenerator, isPathGeneratorDynamic := createTemplateFunc(requestURL.Path, templateFuncMap)
 	methodGenerator, isMethodGeneratorDynamic := NewMethodGeneratorFunc(localRand, methods, templateFuncMap)
@@ -47,7 +48,7 @@ func NewRequestGenerator(
 	cookiesGenerator, isCookiesGeneratorDynamic := NewCookiesGeneratorFunc(localRand, cookies, templateFuncMap)
 
 	bodyTemplateFuncMapData := &BodyTemplateFuncMapData{}
-	bodyTemplateFuncMap := NewDefaultBodyTemplateFuncMap(randSource, bodyTemplateFuncMapData)
+	bodyTemplateFuncMap := NewDefaultBodyTemplateFuncMap(randSource, bodyTemplateFuncMapData, fileCache)
 	bodyGenerator, isBodyGeneratorDynamic := NewBodyGeneratorFunc(localRand, bodies, bodyTemplateFuncMap)
 
 	valuesGenerator := NewValuesGeneratorFunc(values, templateFuncMap)