[[inputs.systemd_units]] scope="user" should support explicitly setting users other than telegraf

[daniel-trnk]

Use Case

Collect systemd unit metrics for systemd units run with a user scope by users other than the telegraf user.

Since #15458, [[inputs.systemd_units]] allows gathering metrics on systemd units run with a user scope in addition to the default system scope. However, it does not allow explicitly setting a user and instead takes the telegraf user.

In most cases, however, relevant services that need to me observed are not run as the telegraf user, but as other users on the system.

Expected behavior

Allow a config like

 scope = "user"
 user = "prod"

Set the user for the systemd user scope unit metrics collection to that user instead of the telegraf user.

Actual behavior

User is set to the current telegraf user.

Additional info

No response