e2e: support AWS Session Token in real provider test (envoyproxy#299)

**Commit Message**
Allow setting AWS session token in the AWS credential file when using
AWS temporary credentials to test AWS Bedrock.

**Related Issues/PRs (if applicable)**

**Special notes for reviewers (if applicable)**
TEST_AWS_SESSION_TOKEN is optional if using static AWS credential.

Signed-off-by: Dan Sun <dsun20@bloomberg.net>
Signed-off-by: Erica Hughberg <erica.sundberg.90@gmail.com>

tests/extproc/real_providers_test.go

@@ -46,7 +46,15 @@ func TestWithRealProviders(t *testing.T) {
 	// Set up credential file for AWS.
 	awsAccessKeyID := getEnvVarOrSkip(t, "TEST_AWS_ACCESS_KEY_ID")
 	awsSecretAccessKey := getEnvVarOrSkip(t, "TEST_AWS_SECRET_ACCESS_KEY")
-	awsCredentialsBody := fmt.Sprintf("[default]\nAWS_ACCESS_KEY_ID=%s\nAWS_SECRET_ACCESS_KEY=%s\n", awsAccessKeyID, awsSecretAccessKey)
+	awsSessionToken := os.Getenv("TEST_AWS_SESSION_TOKEN")
+	var awsCredentialsBody string
+	if awsSessionToken != "" {
+		awsCredentialsBody = fmt.Sprintf("[default]\nAWS_ACCESS_KEY_ID=%s\nAWS_SECRET_ACCESS_KEY=%s\nTEST_AWS_SESSION_TOKEN=%s\n",
+			awsAccessKeyID, awsSecretAccessKey, awsSessionToken)
+	} else {
+		awsCredentialsBody = fmt.Sprintf("[default]\nAWS_ACCESS_KEY_ID=%s\nAWS_SECRET_ACCESS_KEY=%s\n",
+			awsAccessKeyID, awsSecretAccessKey)
+	}
 
 	// Test with AWS Credential File.
 	awsFilePath := t.TempDir() + "/aws-credential-file"