Create sign in/sign out commands

[Tyriar]

The sign out command used to be part of the sync component (Sync: Sign Out), it should be part of core but its own component that manages multiple accounts, for example Account: Sign In, Account Sign Out.

[Tyriar]

@RMacfarlane wouldn't a generic sign in command be better than extension-specific ones? I'd prefer to see in the command palette just "Account: Sign In" rather than "Microsoft: Sign in", "GitHub: Sign in", etc.

[KamasamaK]

@Tyriar And would that generic command then just display a QuickPick of the available providers?

[Tyriar]

Yes, consolidating the sign out logic into core such that provider extensions don't need to worry about it.