Add HTTP keepalive timeout configuration to HTTP-serving components

[achimnol]

The following components have HTTP-serving endpoints:

• Manager
• Storage Proxy
• Web Server (aka Gateway)
• WSProxy (aka App Proxy)
  According to Increase default keepalive_timeout server-side. aio-libs/aiohttp#9285, we should set the HTTP keepalive timeout longer than the reverse-proxy if one is present to avoid races in the reverse-proxy.

aio-libs/aiohttp#9293 (backport of 9285) will be included in aiohttp release 3.11, and I think we should have also an explicit configuration knob to adjust this value as needed by the actual per-site setup.