fix(bigquery): proxy env detection and oauth2 client

Author: Deeven-Seru

internal/sources/bigquery/bigquery.go

@@ -725,7 +725,12 @@ func shouldUseRESTClient(proxyAddress string) bool {
 	if proxyAddress != "" {
 		return true
 	}
-	return os.Getenv("HTTPS_PROXY") != "" || os.Getenv("HTTP_PROXY") != "" || os.Getenv("https_proxy") != "" || os.Getenv("http_proxy") != ""
+	req, err := http.NewRequest(http.MethodGet, "https://bigquery.googleapis.com", nil)
+	if err != nil {
+		return false
+	}
+	proxyURL, err := http.ProxyFromEnvironment(req)
+	return err == nil && proxyURL != nil
 }
 
 func initBigQueryConnection(
@@ -761,6 +766,7 @@ func initBigQueryConnection(
 	if err != nil {
 		return nil, nil, nil, fmt.Errorf("failed to configure base transport: %w", err)
 	}
+	ctx = context.WithValue(ctx, oauth2.HTTPClient, &http.Client{Transport: baseRT})
 
 	if impersonateServiceAccount != "" {
 		// Use default credentials to call IAMCredentials and mint impersonated tokens.
@@ -871,6 +877,7 @@ func newBigQueryClientCreator(
 	if err != nil {
 		return nil, err
 	}
+	ctx = context.WithValue(ctx, oauth2.HTTPClient, &http.Client{Transport: baseRT})
 
 	return func(tokenString string, wantRestService bool) (*bigqueryapi.Client, *bigqueryrestapi.Service, error) {
 		return initBigQueryConnectionWithOAuthToken(ctx, tracer, project, location, name, tokenString, wantRestService, baseRT)
@@ -911,6 +918,7 @@ func initDataplexConnection(
 		if err != nil {
 			return nil, nil, fmt.Errorf("failed to configure base transport: %w", err)
 		}
+		ctx = context.WithValue(ctx, oauth2.HTTPClient, &http.Client{Transport: baseRT})
 
 		var tokenSource oauth2.TokenSource
 		if impersonateServiceAccount != "" {
@@ -1004,6 +1012,7 @@ func newDataplexClientCreator(
 				return nil, err
 			}
 		}
+		ctx = context.WithValue(ctx, oauth2.HTTPClient, &http.Client{Transport: baseRT})
 	}
 	return func(tokenString string) (*dataplexapi.CatalogClient, error) {
 		return initDataplexConnectionWithOAuthToken(ctx, project, userAgent, tokenString, useREST, baseRT)