SDK / exoplayer

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