Provide an authentication and secrets mechanism for database access

[jonsequitur]

In order to prevent people from having to paste connection strings into notebooks, we should have proper authentication support. This could include:

• Aliases for connection strings that can be referenced from with the #!connect magic command.
• AAD auth flow.

[jcmrva]

It would be great if we could use the dotnet user-secrets tool with notebooks.

[brettfo]

Thinking a little more broadly than databases, could we possibly get auth tokens via RequestInput and a specific value for inputTypeHint? VS Code extensions can provide authenticators, so if inputTypeHint was "token:github", the VS Code extension could get the GitHub auth provider (if installed) and try to get a token directly from that. We even have a good fall-back story, where if the requested auth provider isn't installed and we can't automatically get a token, we can simply prompt the user for a value. In the Jupyter Lab case we could simply fall back to the Jupyter method of prompting for input, and in the console/automation case, we could pull the value from a command line argument.

[jonsequitur]

The input type hint system was intended to be used this way, so now it's hopefully just a matter of building the providers.

[jonsequitur]

This work depends on #3567.