BlindSignAuth: Refactoring to support future Android BlindSignMessageInterface impl.

- Rename BlindSignHttpInterface to BlindSignMessageInterface
- Change BSA::GetTokens oauth_token parameter to std::optional<std::string>, since Android BSA callers will not have an OAuth token to pass in
- Change BlindSignMessageInterface::DoRequest authorization_header to std::optional<std::string>
- Stop setting oauth_token field in AuthAndSignRequest (field is unused for IP Protection and will be deprecated)

PiperOrigin-RevId: 614767846

build/source_list.json

diff --git a/build/source_list.json b/build/source_list.json
index 6bebbca..5c154ef 100644
--- a/build/source_list.json
+++ b/build/source_list.json
@@ -1615,8 +1615,8 @@
     "quiche/blind_sign_auth/blind_sign_auth.h",
     "quiche/blind_sign_auth/blind_sign_auth_interface.h",
     "quiche/blind_sign_auth/blind_sign_auth_protos.h",
-    "quiche/blind_sign_auth/blind_sign_http_interface.h",
     "quiche/blind_sign_auth/blind_sign_http_response.h",
+    "quiche/blind_sign_auth/blind_sign_message_interface.h",
     "quiche/blind_sign_auth/cached_blind_sign_auth.h",
     "quiche/blind_sign_auth/test_tools/mock_blind_sign_auth_interface.h",
     "quiche/blind_sign_auth/test_tools/mock_blind_sign_http_interface.h"