Add type defs to Dictionary class for ease of use with templated code

PiperOrigin-RevId: 959076878

quiche/common/structured_headers.h

diff --git a/quiche/common/structured_headers.h b/quiche/common/structured_headers.h
index a36a497..579b95f 100644
--- a/quiche/common/structured_headers.h
+++ b/quiche/common/structured_headers.h
@@ -221,6 +221,9 @@
  public:
   using iterator = std::vector<DictionaryMember>::iterator;
   using const_iterator = std::vector<DictionaryMember>::const_iterator;
+  using key_type = std::string;
+  using mapped_type = ParameterizedMember;
+  using value_type = std::pair<const std::string, ParameterizedMember>;
 
   Dictionary();
   Dictionary(const Dictionary&);