title: Use multiple JWT Bearer Authentication
body: <p>Is it possible to support multiple JWT Token issuers in ASP.NET Core 2?
  I want to provide an API for external service and I need to use two sources of JWT tokens - Firebase and custom JWT token issuers. In ASP.NET core I can set the JWT authentication for Bearer auth scheme, but only for one Authority:</p>
  
  <pre><code>  services
          .AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
          .AddJwtBearer(options =&gt;
          {
              options.Authority = "https://securetoken.google.com/my-firebase-project"
              options.TokenValidationParameters = new TokenValidationParameters
                  {
                      ValidateIssuer = true,
                      ValidIssuer = "my-firebase-project"
                      ValidateAudience = true,
                      ValidAudience = "my-firebase-project"
                      ValidateLifetime = true
                  };
          }
  </code></pre>
  
  <p>I can have multiple issuers and audiences, but I can't set several Authorities.</p>