Fix AWS session token support

[JacobHenner]

AWS session token authentication was inadvertently broken in a refactor of AWS auth handling. Restoring support for session tokens. This fixes a regression that broke the feature introduced in #2573.

Checklist

• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
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Fixes #5156

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[JacobHenner]

I intend to add tests later this week, which should prevent a similar regression from recurring.

[JacobHenner]

I had considered adding tests along the lines of:

package scalers

import (
	"context"
	"testing"

	"github.com/stretchr/testify/assert"
)

const (
	testAWSCommonRoleArn         = "none"
	testAWSCommonAccessKeyID     = "none"
	testAWSCommonSecretAccessKey = "none"
	testAWSCommonSessionToken    = "none"
)

func TestCredentialPropagation(t *testing.T) {
	ctx := context.Background()
	awsRegion := "eu-west-1"
	awsAuthorization := awsAuthorizationMetadata{
		awsAccessKeyID:     testAWSCommonAccessKeyID,
		awsSecretAccessKey: testAWSCommonSecretAccessKey,
		awsSessionToken:    testAWSCommonSessionToken,
	}
	awsConfig, err := getAwsConfig(ctx, awsRegion, awsAuthorization)
	if err != nil {
		t.Errorf("unexpected error in getAwsConfig: %v", err)
	}
	creds, err := awsConfig.Credentials.Retrieve(ctx)
	if err != nil {
		t.Errorf("unexpected error while retrieving credentials: %v", err)
	}
	assert.Equal(t, testAWSCommonAccessKeyID, creds.AccessKeyID)
	assert.Equal(t, testAWSCommonSecretAccessKey, creds.SecretAccessKey)
	assert.Equal(t, testAWSCommonSessionToken, creds.SessionToken)

}

But awsConfig.Credentials.Retrieve(ctx) fails, as it errors while attempting to refresh credentials as a result of using non-valid test values in awsAuthorizationMetadata.

[JorTurFer]

/run-e2e aws
Update: You can check the progress here

[JorTurFer]

Thanks for the fix @JacobHenner
Can we cover this scenario somehow using e2e tests?