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.
`auth.username`	(string) Username for HTTP Basic authentication to the artifact registry. Must be set together with `auth.password`, and is mutually exclusive with `auth.api_key`.
`auth.password`	(string) Password for HTTP Basic authentication to the artifact registry. Must be set together with `auth.username`, and is mutually exclusive with `auth.api_key`.
`auth.api_key`	(string) API key sent in the `Authorization: ApiKey <api-key>` header. Mutually exclusive with `auth.username` and `auth.password`. If both are configured, `auth.api_key` takes precedence.
`auth.headers`	(list) Custom HTTP headers sent to the artifact registry on every request. Each entry is an object with `key` and `value` fields.