revert(azure): revert removal (#3890)

* Revert "fix(sidebar): show docker sidebar when needed (#3852)"

This reverts commit 59da17dde4.

* Revert "refactor(azure): remove Azure ACI endpoint support (#3803)"

This reverts commit 493de20540.

api/http/client/client.go

@@ -2,9 +2,12 @@ package client
 
 import (
 	"crypto/tls"
+	"encoding/json"
+	"fmt"
 	"io/ioutil"
 	"log"
 	"net/http"
+	"net/url"
 	"strings"
 	"time"
 
@@ -16,6 +19,55 @@ const (
 	defaultHTTPTimeout       = 5
 )
 
+// HTTPClient represents a client to send HTTP requests.
+type HTTPClient struct {
+	*http.Client
+}
+
+// NewHTTPClient is used to build a new HTTPClient.
+func NewHTTPClient() *HTTPClient {
+	return &HTTPClient{
+		&http.Client{
+			Timeout: time.Second * time.Duration(defaultHTTPTimeout),
+		},
+	}
+}
+
+// AzureAuthenticationResponse represents an Azure API authentication response.
+type AzureAuthenticationResponse struct {
+	AccessToken string `json:"access_token"`
+	ExpiresOn   string `json:"expires_on"`
+}
+
+// ExecuteAzureAuthenticationRequest is used to execute an authentication request
+// against the Azure API. It re-uses the same http.Client.
+func (client *HTTPClient) ExecuteAzureAuthenticationRequest(credentials *portainer.AzureCredentials) (*AzureAuthenticationResponse, error) {
+	loginURL := fmt.Sprintf("https://login.microsoftonline.com/%s/oauth2/token", credentials.TenantID)
+	params := url.Values{
+		"grant_type":    {"client_credentials"},
+		"client_id":     {credentials.ApplicationID},
+		"client_secret": {credentials.AuthenticationKey},
+		"resource":      {"https://management.azure.com/"},
+	}
+
+	response, err := client.PostForm(loginURL, params)
+	if err != nil {
+		return nil, err
+	}
+
+	if response.StatusCode != http.StatusOK {
+		return nil, portainer.ErrAzureInvalidCredentials
+	}
+
+	var token AzureAuthenticationResponse
+	err = json.NewDecoder(response.Body).Decode(&token)
+	if err != nil {
+		return nil, err
+	}
+
+	return &token, nil
+}
+
 // Get executes a simple HTTP GET to the specified URL and returns
 // the content of the response body. Timeout can be specified via the timeout parameter,
 // will default to defaultHTTPTimeout if set to 0.