Fix -Wc++11-narrowing in quiche.

char is a signed type and cannot hold values >=0x80, so storing these
results in a warning.  Instead, store runs of bytes as uint8_t, and
explicitly cast to char* where needed.  See https://crbug.com/1216696.

This CL is generated from
https://quiche-review.googlesource.com/c/quiche/+/10700.  Unfortunately QUICHE
currently does not accept external contributions directly.

PiperOrigin-RevId: 378433199