Add the new `size()` method to ConstHeaderApi and its implementations.

This change introduces a virtual `size()` method to the `ConstHeaderApi`. Implementations in `BalsaHeaders`, `WrappedConstEnvoyHeaders`, and `HeaderApiWrapper` are updated to support this.

We will then use this to call this method from the DoS eval, which uses `ConstHeaderApi`.

PiperOrigin-RevId: 936415990

quiche/balsa/balsa_headers.h

diff --git a/quiche/balsa/balsa_headers.h b/quiche/balsa/balsa_headers.h
index d7e90cf..da57922 100644
--- a/quiche/balsa/balsa_headers.h
+++ b/quiche/balsa/balsa_headers.h
@@ -1015,7 +1015,7 @@
 
   bool IsEmpty() const override;
 
-  size_t size() const {
+  size_t size() const override {
     if (header_lines_removed_ > header_lines_.size()) {
       return 0;
     }

quiche/balsa/header_api.h

diff --git a/quiche/balsa/header_api.h b/quiche/balsa/header_api.h
index e607172..f9f0a17 100644
--- a/quiche/balsa/header_api.h
+++ b/quiche/balsa/header_api.h
@@ -44,6 +44,9 @@
   // Determine whether the headers are empty.
   virtual bool IsEmpty() const = 0;
 
+  // Returns the number of headers.
+  virtual size_t size() const = 0;
+
   // Returns the header entry for the first instance with key |key|
   // If header isn't present, returns absl::string_view().
   virtual absl::string_view GetHeader(absl::string_view key) const = 0;