[hailtop] allow configuration of default HTTP timeout

[danking]

Until we have a mechanism to infer the correct timeout based on network conditions, this provides an escape hatch for users on flaky network connections such as wifi.

[danking]

Confirmed that the hailctl bit works:

(base) dking@wm28c-761 hail % hailctl config set http/timeout_in_seconds 1234s
Error: bad value '1234s' for parameter <ConfigVariable.HTTP_TIMEOUT_IN_SECONDS: 'http/timeout_in_seconds'> should be a float or an int like 42.42 
or 42
(base) dking@wm28c-761 hail % hailctl config set http/timeout_in_seconds 42   
(base) dking@wm28c-761 hail % hailctl config set http/timeout_in_seconds 42.0
(base) dking@wm28c-761 hail % hailctl config set http/timeout_in_seconds 60  
(base) dking@wm28c-761 hail % cat ~/.config/hail/config.ini                
[query]
backend = spark
jar_url = gs://hail-query-ger0g/jars/dking/uk4prwgezgva/5fc88d5a4b614454004226f5c77ea72efee1e38f.jar

[batch]
remote_tmpdir = gs://1-day/
billing_project = hail
backend = service

[aiocloudflare]

[global]
domain = hail.is

[gcs_requester_pays]
project = broad-ctsa

[http]
timeout_in_seconds = 60