Deprecate `user` config in the plugin.

[mashhurs]

Description

In the plugin we use username config and when we directly involved http-output, we applied username to http-output plugin. However, with HA support work, we are going to directly use http-mixin which has a user option for user name.
With a standardization works, for the long term, we would like to keep username and deprecate user.
This PR introduces multiple additional useful APIs such as with_required_aliases, with_dependent_aliases and with_conflicting_aliases APIs. And, we can use with_conflicting_aliases for our case.

[mashhurs]

Applied this change with this commit, closing.