lib/awsapi: add support for named AWS profile to ec2_sd_config

Add support for named AWS profiles in ec2_sd_config, matching Prometheus behavior.

Example:

```text
~/.aws/config:
[profile account-one]
source_profile = root
role_arn = arn:aws:iam::000000000001:role/prometheus
```

```yaml
scrape config:
- job: ec2
  ec2_sd_configs:
    - profile: account-one
```

Fixes https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1685

app/vmagent/remotewrite/client.go

@@ -290,7 +290,7 @@ func getAWSAPIConfig(argIdx int) (*awsapi.Config, error) {
 	accessKey := awsAccessKey.GetOptionalArg(argIdx)
 	secretKey := awsSecretKey.GetOptionalArg(argIdx)
 	service := awsService.GetOptionalArg(argIdx)
-	cfg, err := awsapi.NewConfig(ec2Endpoint, stsEndpoint, region, roleARN, accessKey, secretKey, service)
+	cfg, err := awsapi.NewConfig(ec2Endpoint, stsEndpoint, region, roleARN, accessKey, secretKey, service, "")
 	if err != nil {
 		return nil, err
 	}