Fix csrf_detected for concurrent requests

[nickstanish]

Relates to #103

Went with a hash approach instead of array to better work alongside the pkce verifier

Added tests and verified in my application. Without these changes this test fails:

       Expected "/auth/failure?message=csrf_detected&strategy=auth0"
          to eq "http://www.example.com/dashboard"
     # ./spec/requests/oauth_spec.rb:194:in 'block (5 levels) in <main>'

        it 'fails with csrf_detected when multiple tabs initiate auth0 flows simultaneously' do
          # Tab 1 initiates
          post '/auth/auth0'
          expect(response).to have_http_status(:found)
          auth0_redirect_uri_1 = Addressable::URI.parse(response.headers['Location'])
          query_params_1 = CGI.parse(auth0_redirect_uri_1.query).transform_values(&:first)
          state_1 = query_params_1['state']

          # Then Tab 2 follows
          post '/auth/auth0'
          expect(response).to have_http_status(:found)
          auth0_redirect_uri_2 = Addressable::URI.parse(response.headers['Location'])
          query_params_2 = CGI.parse(auth0_redirect_uri_2.query).transform_values(&:first)
          state_2 = query_params_2['state']

          allow_any_instance_of(OmniAuth::Strategies::Auth0).to receive(:raw_info).and_return({
            sub: SecureRandom.uuid,
            name: Faker::Name.name,
            email: Faker::Internet.email,
          })

          stub_request(:post, 'https://auth0.local/oauth/token')
            .to_return(
              status: 200,
              body: {
                access_token: 'mock_access_token',
                token_type: 'Bearer',
                expires_in: 86_400,
                id_token: 'mock_id_token',
              }.to_json,
              headers: { 'Content-Type' => 'application/json' },
            )

          # Tab 1 callback completes
          get '/auth/auth0/callback', params: { state: state_1, code: 'mock_code_1' }
          expect(response).to have_http_status(:found)
          expect(response.headers['Location']).to eq('http://www.example.com/dashboard')

          # Tab 2 callback completes
          get '/auth/auth0/callback', params: { state: state_2, code: 'mock_code_1' }
          expect(response).to have_http_status(:found)
          expect(response.headers['Location']).to eq('http://www.example.com/dashboard')
        end