deprecate formatAtsign() in AtUtils and move the functionality to fixAtSign()

[srieteja]

Is your feature request related to a problem? Please describe.

There are currently two methods:

• formatAtsign(): which prefixes an atsign with '@' when it's missing one
• fixAtsign(): which performs all other checks/fixes atsign to match a standard format

To end up with a correctly formatted atsign one needs to use the two of these methods and it can be confusing.

Describe the solution you'd like

Deprecate formatAtsign() and move it's functionality to fixAtsign(), which allows a simpler and a more cleaner mechanism to format atsign's

Task checklist

• deprecate formatAtsign()
• move it's functionality to fixAtSign()
• integration test with all other packages e.g. at_server, at_client, at_onboarding_cli, etc
• publish at_utils with this change