MINOR: log: add app_log_raw() and send_log_raw() for binary-safe logging

MINOR: log: add app_log_raw() and send_log_raw() for binary-safe logging

app_log() and send_log() build the message with vsnprintf(), which stops
at the first NUL byte and therefore cannot emit an arbitrary binary
payload.

Add two variants that pass a pre-built <msg> of <len> bytes straight to
__send_log() without formatting it, so embedded NUL bytes are preserved:

  * app_log_raw()  : takes an explicit list of loggers and a tag
  * send_log_raw() : derives both from a proxy

The send path still strips trailing LF / NUL bytes (kept for the legacy
text logs), so the message must be self-terminating by its own encoding
and must not rely on a meaningful trailing '\n' or NUL.