fix(http): time out stalled transactions without killing switched protocols

[nickanderson]

Backport of #2008 to 5.0.x. Both 5.0.5 and 5.1.0 were verified affected, so upgrading is
not a workaround.

A server that accepts a request and then never answers left the response read outstanding
indefinitely, wedging the client silently — alive, no crash, no log output, recoverable only
by restart. The gap was ReadHeader(): AsyncReadNextBodyPart already had a timer, but
nothing bounded the wait for response headers.

This arms Beast's stream timeout before each operation of a transaction and disarms it at
the 101 Switching Protocols handover. That handover is what the previous attempt
(d8ddf6da / 4aa01ee8) got wrong: it left the timeout armed, and Beast's timeout_handler
calls close() on expiry regardless of who owns the socket by then, which killed
mender-connect's forwarded WebSocket after five minutes and got the fix reverted under
MEN-9433.

Connect is intentionally not covered — asio::async_connect runs on lowest_layer(), the
raw socket, which bypasses the stream timeout entirely.

Why this touches an extra file versus master

src/common/common.hpp is included here to bring common::def_value over from master; 5.0.x
only has def_bool. It is needed for the new ClientConfig::stream_timeout_seconds field.

A plain default member initializer would have been smaller but is wrong on this branch:
cmake/build_mode.cmake compiles Debug with -std=c++11 deliberately, and under C++11 a
default member initializer stops ClientConfig being an aggregate — which breaks
http::ClientConfig client_config {"server.localhost.crt"} in the HTTPS tests. That is the
same reason def_bool already exists.

Tests

Both are red/green verified — each fails when its own half of the fix is removed:

test	without the fix
HttpTest.TestStalledResponseTimesOut	hangs to the event-loop watchdog (5006 ms)
HttpForwarderTests.ProtocolSwitchSurvivesIdlePastStreamTimeout	fails at 1219 ms — MEN-9433 exactly

The forwarder already had five ProtocolSwitch* tests, but all exercise immediate data flow;
none left the connection idle, which is why MEN-9433 escaped.

Built on 5.0.x in Debug (the C++11 path, so the aggregate trap above is exercised);
http_test and http_forwarder_test pass 58/58 via ctest.

End-to-end on 5.0.5 with a stalled deployments/next and the reporting customer's state
scripts: unpatched logs nothing for 420 s, patched logs
Error while polling for deployment: The operation timed out at exactly t+300 s, runs the
SyncError/IdleEnter scripts, and resumes polling.

Ticket: ME-731

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[mender-test-bot]

Merging these commits will result in the following changelog entries:

Changelogs

mender (cherry-5.0.x-ME-731-http-stream-timeout)

New changes in mender since 5.0.x:

Bug Fixes

• Fixed the client hanging indefinitely when a server accepts a request but
  never sends a response, which left the device unmanageable until it was restarted.
  (ME-731, MEN-9433)