Adopt `getAccounts` API in GitHub Authentication (#221224)

This allows the GitHub Auth provider to take in a account per this proposal:

https://github.com/microsoft/vscode/blob/417dddb83f3536479f88c40e1d78edf1136a5ae5/src/vscode-dts/vscode.proposed.authGetSessions.d.ts#L35-L69

Additionally, this makes an additional small change that allows `clearSessionPreference` to work in single-account auth providers.