Do not block startup if OIDC provider cannot be created

Fixes #1358

If a provider is down, this will prevent the service from starting. We
now log an error if the provider is down. The service will try to fetch
the provider during the next request where that provider is needed.

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>

pkg/config/config.go

@@ -229,10 +229,11 @@ func (fc *FulcioConfig) prepare() error {
 		defer cancel()
 		provider, err := oidc.NewProvider(ctx, iss.IssuerURL)
 		if err != nil {
-			return fmt.Errorf("provider %s: %w", iss.IssuerURL, err)
+			log.Logger.Errorf("error creating provider for issuer URL %q: %v", iss.IssuerURL, err)
+		} else {
+			cfg := &oidc.Config{ClientID: iss.ClientID}
+			fc.verifiers[iss.IssuerURL] = []*verifierWithConfig{{provider.Verifier(cfg), cfg}}
 		}
-		cfg := &oidc.Config{ClientID: iss.ClientID}
-		fc.verifiers[iss.IssuerURL] = []*verifierWithConfig{{provider.Verifier(cfg), cfg}}
 	}
 
 	cache, err := lru.New2Q(100 /* size */)