* Allow specifying SIP username for outgoing calls
Allow configuring a SIP username to be sent in outgoing call requests to
identify the home assistant source endpoint.
* Remove advanced options section
* Add test for removing user
* Allow unsetting SIP user
Make previous SIP user value a suggested value rather than default to
allow unsetting by submitting an empty value in the form.
* Remove unnecessary checks
Remove user check from main flow and remove none or empty check.
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