Make kExpectedExtensionTypes type array<const uint16_t>

That's because the callee ValidateExtensionsOrderAndValues arg is
typed as absl::Span<const uint16_t>.
This CL would unblock Chromium roll.

PiperOrigin-RevId: 777511032

quiche/blind_sign_auth/blind_sign_auth.cc

diff --git a/quiche/blind_sign_auth/blind_sign_auth.cc b/quiche/blind_sign_auth/blind_sign_auth.cc
index 8c0415d..7950dd9 100644
--- a/quiche/blind_sign_auth/blind_sign_auth.cc
+++ b/quiche/blind_sign_auth/blind_sign_auth.cc
@@ -45,7 +45,7 @@
 constexpr absl::string_view kIssuerHostname =
     "https://ipprotection-ppissuer.googleapis.com";
 constexpr size_t kExpectedExtensionTypesSize = 5;
-constexpr std::array<uint16_t, kExpectedExtensionTypesSize>
+constexpr std::array<const uint16_t, kExpectedExtensionTypesSize>
     kExpectedExtensionTypes = {0x0001, 0x0002, 0xF001, 0xF002, 0xF003};
 
 using privacy::ppn::AuthAndSignRequest;