This commit adds two new `agentsdk` functions, `StartupLogsSender` and
`StartupLogsWriter` that can be used by any client looking to send
startup logs to coderd.
We also refactor the `agent` to use these new functions.
As a bonus, agent startup logs are separated into "info" and "error"
levels to separate stdout and stderr.
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Co-authored-by: Marcin Tojek <mtojek@users.noreply.github.com>
