avformat/http: support retry on connection error

avformat/http: support retry on connection error

Add 2 new options:
- reconnect_on_http_error - a list of http status codes that should be
    retried. the list can contain explicit status codes / the strings
    4xx/5xx.
- reconnect_on_network_error - reconnects on arbitrary errors during
    connect, e.g. ECONNRESET/ETIMEDOUT

the retry employs the same exponential backoff logic as the existing
reconnect/reconnect_at_eof flags.

related tickets:
https://trac.ffmpeg.org/ticket/6066
https://trac.ffmpeg.org/ticket/7768

Signed-off-by: Marton Balint <cus@passwd.hu>