When constructing the client, and setting the User-Agent, care must be
taken to apply the header in the right location, as custom headers can
be set in the CLI configuration, and merging these custom headers should
not override the User-Agent header.
This patch adds a dedicated `WithUserAgent()` option, which stores the
user-agent separate from other headers, centralizing the merging of
other headers, so that other parts of the (CLI) code don't have to be
concerned with merging them in the right order.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
