Move RFC 9000 variable length integer encoding from QUIC to QUICHE

Since BHTTP uses these, and we're starting BHTTP in QUICHE, we need to move the read/write functions for varints to QUICHE. While I was in there, I improved some comments to refer to the published RFC, and removed a few ODR violations.

This CL is a no-op, it does not modify any existing functionality.

PiperOrigin-RevId: 467990159
diff --git a/quiche/quic/core/quic_stream_test.cc b/quiche/quic/core/quic_stream_test.cc
index 51b25a5..1bec69f 100644
--- a/quiche/quic/core/quic_stream_test.cc
+++ b/quiche/quic/core/quic_stream_test.cc
@@ -346,8 +346,8 @@
               connection_->transport_version(), PACKET_8BYTE_CONNECTION_ID,
               PACKET_0BYTE_CONNECTION_ID, !kIncludeVersion,
               !kIncludeDiversificationNonce, PACKET_4BYTE_PACKET_NUMBER,
-              VARIABLE_LENGTH_INTEGER_LENGTH_0,
-              VARIABLE_LENGTH_INTEGER_LENGTH_0, 0u);
+              quiche::VARIABLE_LENGTH_INTEGER_LENGTH_0,
+              quiche::VARIABLE_LENGTH_INTEGER_LENGTH_0, 0u);
   connection_->SetMaxPacketLength(length);
 
   EXPECT_CALL(*session_, WritevData(kTestStreamId, _, _, _, _, _))
@@ -431,8 +431,8 @@
               connection_->transport_version(), PACKET_8BYTE_CONNECTION_ID,
               PACKET_0BYTE_CONNECTION_ID, !kIncludeVersion,
               !kIncludeDiversificationNonce, PACKET_4BYTE_PACKET_NUMBER,
-              VARIABLE_LENGTH_INTEGER_LENGTH_0,
-              VARIABLE_LENGTH_INTEGER_LENGTH_0, 0u);
+              quiche::VARIABLE_LENGTH_INTEGER_LENGTH_0,
+              quiche::VARIABLE_LENGTH_INTEGER_LENGTH_0, 0u);
   connection_->SetMaxPacketLength(length);
 
   EXPECT_CALL(*session_, WritevData(_, _, _, _, _, _))