Support provider-agnostic cloud OIDC tokens

[eekdood]

This updates the device-side cloud registration/session logic to accept provider-agnostic OIDC fields from the cloud API while preserving compatibility with the existing Google-specific fields.

This pairs with the cloud-api change that makes the OIDC provider configurable. jetkvm/cloud-api#64

Changes

• Accept provider-agnostic cloud registration fields:
  • oidcToken
  • oidcClientId
  • oidcIssuer
• Preserve legacy fallback fields:
  • oidcGoogle
  • clientId
• Store provider-aware OIDC identity data:
  • oidc_issuer
  • oidc_identity
• Keep populating google_identity for backward compatibility.
• Verify cloud session OIDC tokens against the stored issuer.
• Fall back to Google issuer/identity behavior for legacy configurations.
• Update WebRTC session auth to accept OidcToken while preserving OidcGoogle.
• Update the adoption UI to consume and forward the new provider-agnostic fields.

Compatibility

Existing Google-based cloud registrations continue to work through the legacy fields and fallback issuer. This PR intentionally avoids renaming existing Google-specific config fields beyond adding the new provider-agnostic fields, so the behavioral change stays focused and low-risk.

Testing

• gofmt
• git diff --check
• npm run build:device
• Fork GitHub Actions passed:
  • lint
  • build/test
• Added tests for:
  • provider-agnostic field precedence
  • legacy Google field fallback
  • issuer/client/subject identity format
  • missing OIDC identity claims
  • fake cloud adoption using a local OIDC issuer/JWKS and signed ID token

[CLAassistant]

All committers have signed the CLA.

[eekdood]

This should address some of the concerns raised in #99 and #520