Add timeout to `Client.container.exec_run()`

It's unusually difficult to add a timeout to this method. This is important if we want to exec a command that may have an unreasonable wait time.

I have tried accessing the underlying socket to add a timeout, but this doesn't seem to have any effect. My initial (not-working) attempt is this:

from docker.utils.socket import consume_socket_output, demux_adaptor, frames_iter

# This exec will not finish for a very long time
_, socket = self.container.exec_run(
    ["sleep", "999999"], stdout=True, stderr=True, demux=True, socket=True
)

# Access the underlying socket to add a timeout
socket._sock.settimeout(5)

# Use internal methods to read from the socket normal
gen = frames_iter(socket, tty)
gen = (demux_adaptor(*frame) for frame in gen)
stdout, stderr = consume_socket_output(gen, demux=True)
return (stdout or stderr).decode()

System details:

docker==4.3.0
Python 3.7.6
Client: Docker Engine - Community
 Version:           19.03.11
 API version:       1.40
 Go version:        go1.13.10
 Git commit:        42e35e61f3
 Built:             Mon Jun  1 09:12:23 2020
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          19.03.11
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.13.10
  Git commit:       42e35e61f3
  Built:            Mon Jun  1 09:10:55 2020
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.2.13
  GitCommit:        7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc:
  Version:          1.0.0-rc10
  GitCommit:        dc9208a3303feef5b3839f4323d9beb36df0a9dd
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

Start at Client.container.exec_run() and trace how its returned socket is consumed through docker.utils.socket, including frames_iter, demux_adaptor, and consume_socket_output. Check how a timeout could be exposed without relying on the underlying socket setting. Done means callers can limit a long-running exec and the behavior is covered by the relevant tests.