Enable signin using popup modals instead of redirections

[FedericoAmura]

Description

This PR adds a signin method that handles authenticating with an oauth provider using a popup modal instead of a redirect and two steps
This feature requires lit-login-server PR#4

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Manually built the SDK, dropped built files in the wagmi connector demo and used Google and Discord auth modal flows there. Also running a local version of lit login server

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[Ansonhkg]

This looks good to me. Just so you're aware, we also provide a callback in the sign-in function to handle their own way of signing in, e.g., a pop-up.

js-sdk/packages/lit-auth-client/src/lib/providers/GoogleProvider.ts

Line 36 in 130edda

public async signIn(callback?: (url: string) => void): Promise<void> {