[Epic] Support GitHub OIDC authentication

[lgecse]

Currently, joining the Directory federation requires manual creation of join tokens, which limits accessibility and scalability. We want to enable any user to authenticate and join the federation—even from a local machine—using standard OIDC identity providers (GitHub, Google, etc.).

┌─────────────────┐     ┌─────────────────┐     ┌──────────────────┐     ┌─────────────────┐
│   OIDC Provider │     │   SPIRE Agent   │     │   SPIRE Server   │     │    Directory    │
│   (GitHub)      │     │   (User Node)   │     │                  │     │                 │
└────────┬────────┘     └────────┬────────┘     └────────┬─────────┘     └────────┬────────┘
         │                       │                       │                        │
         │ 1. Get OIDC token     │                       │                        │
         │◄──────────────────────│                       │                        │
         │                       │                       │                        │
         │ 2. Present token for  │                       │                        │
         │    node attestation   │                       │                        │
         │                       │──────────────────────►│                        │
         │                       │                       │                        │
         │                       │ 3. Issue SVID         │                        │
         │                       │◄──────────────────────│                        │
         │                       │                       │                        │
         │                       │ 4. Join federation with SVID                   │
         │                       │───────────────────────────────────────────────►│

Goals

• Remove manual join token provisioning
• Enable self-service federation onboarding
• Support Github OIDC provider (later we can explore AWS, Google, Azure AD)