Embed: Implement Web2 OAuth Flows

[Jkd-eth]

User Story:

As a user of the Embed solution,
I want Web2 OAuth flows to be integrated,
So that I can authenticate using popular services via a popup flow.

Depends on:

• Pull Platform metadata from embed service #3205

Acceptance Criteria

GIVEN the Embed solution is deployed,
WHEN a user opts to claim a Web2 stamp for which OAuth is required,
THEN a popup flow facilitates the authentication and verification process.

Update the implementation for Google, Discord, Github & Coinbase.

Tech Notes

• need to add the /challenge and /verify API endpoints in the embed service
  • on /verify stamps shall be saved to the DB similar like in the auto verification flow
• break out the secrets & env configuration to its own 1P entry and add the service & popup items there?
• atm, only LinkedIn works, we need to add support for GH, Discord and Google
• deployment of the popup page via GHA + pulumi

Product & Design Links:

N/A

Open Questions:

Notes/Assumptions: