chore: Migrate at_mono module repos

[XavierChanth]

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

While at_mono "works" it is currently difficult to run tests from the repo, and other forms of automation, this will be especially true as we begin to add packages to at_mono.

Describe the solution you'd like

First migrate all packages to follow the new dart repo specification:

• at_server : atsign-foundation/at_server#975
• at_client_sdk : atsign-foundation/at_client_sdk#757
• at_widgets : atsign-foundation/at_widgets#548
• at_libraries : atsign-foundation/at_libraries#240
• at_tools : atsign-foundation/at_tools#247
• at_app : atsign-foundation/at_app#211
• at_services : atsign-foundation/at_services#6

Then refactor workflows into composite actions, so that they may be reused in at_mono's workflows:

• at_server
• at_client_sdk
• at_widgets
• at_libraries
• at_tools
• at_app
• at_services

Describe alternatives you've considered

No response

Additional context

No response

[XavierChanth]

@cpswan @gkc the three open PRs listed are ready to be merged over this weekend

[XavierChanth]

Minimal progress was made in PR 50, moving over to PR 51.

[XavierChanth]

Backlogging to focus on docs site