`expires_in` in OAuth2 Token is too huge

[YoitoFes]

According to RFC6749, expires_in in OAuth2 Token is the lifetime in seconds of the access token.

• https://www.rfc-editor.org/rfc/rfc6749#section-4.2.2

But mockoidc set MockOIDC.AccessTTL directly to expires_in, making expires_in huge; e.g. if AccessTTL = 10 * time.Seconds (1000000000 int64), expires_in becomes 10000000000 in seconds.

• https://github.com/oauth2-proxy/mockoidc/blob/main/handlers.go#L183

  tr := &tokenResponse{
  RefreshToken: req.Form.Get("refresh_token"),
  TokenType:    "bearer",
  ExpiresIn:    m.AccessTTL,
  }