Configure download settings for standalone Elastic Agent upgrades

The agent.download section of the elastic-agent.yml config file contains settings for where to download and store artifacts used for Elastic Agent upgrades.

Setting	Description
`sourceURI`	(string) Path to the location of artifacts used during Elastic Agent upgrade.
`target_directory`	(string) Path to the directory where download artifacts are stored.
`timeout`	(string) The HTTP request timeout in seconds for the download package attempt.
`install_path`	(string) The location of installed packages and programs, as well as program specifications.
`retry_sleep_init_duration`	(string) The duration in seconds to sleep for before the first retry attempt.