Authentication sequence in RTSP:
- Server replies "Unauthorized" to our DESCRIBE request, and includes the
necessary information (i.e. realm, digest nonce, etc) in WWW-Authenticate
header
- After `RtspClient` receives the response, we
- Parse the WWW-Authenticate header, stores the auth info. The info is saved
for all further RTSP requests (that all need to carry authorization headers)
- send the second DESCRIBE request with the Authorization header.
#minor-release
PiperOrigin-RevId: 376116302
