FwupdBackend: Set user agent from client

Ensures it include any information fwupd expects/sets, such as the fwupd version, distro version, in addition to our package version.

BUG: 486726

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User agent in my case is

plasma-discover/6.0.80 (Linux x86_64 6.5.0-26-generic; de-DE; KDE neon 22.04) fwupd/1.9.15

fwupd_client_get_user_agent was introduced in 1.5.2 which should be old enough to use even on LTS.

Haven’t tested actual downloading of updates, yet, though.

@apol

5.27 please