feat: lacework_integration_ecr support for IAM Roles

We now support integrating ECR using IAM Roles instead of keys, though,
we will continue to support access keys.

To create an ECR integration with the new IAM role authenticatino method
```
provider "lacework" {}

resource "lacework_integration_ecr" "iam_role" {
  name            = "ECR using IAM Role"
  registry_domain = "YourAWSAccount.dkr.ecr.YourRegion.amazonaws.com"
  credentials {
    role_arn    = "arn:aws:iam::1234567890:role/lacework_iam_example_role"
    external_id = "12345"
  }
}
```

To create an ECR integration with the old authentication method, using
Access Keys:
```
provider "lacework" {}

resource "lacework_integration_ecr" "access_key" {
  name            = "ECR using Access Keys"
  registry_domain = "YourAWSAccount.dkr.ecr.YourRegion.amazonaws.com"
  credentials {
    access_key_id     = "AWS123abcAccessKeyID"
    secret_access_key = "AWS123abc123abcSecretAccessKey0000000000"
  }
}
```

Signed-off-by: Salim Afiune Maya <afiune@lacework.net>

examples/resource_lacework_integration_ecr/main.tf

@@ -1,12 +1,24 @@
 provider "lacework" {}
 
-resource "lacework_integration_ecr" "example" {
-  name              = "ERC Example"
-  registry_domain   = "YourAWSAccount.dkr.ecr.YourRegion.amazonaws.com"
-  access_key_id     = "AWS123abcAccessKeyID"
-  secret_access_key = "AWS123abc123abcSecretAccessKey0000000000"
-  limit_by_tag      = "dev*"
-  limit_by_label    = "*label"
-  limit_by_repos    = "my-repo,other-repo"
-  limit_num_imgs    = 10
+resource "lacework_integration_ecr" "iam_role" {
+  name            = "ECR using IAM Role"
+  registry_domain = "YourAWSAccount.dkr.ecr.YourRegion.amazonaws.com"
+  credentials {
+    role_arn    = "arn:aws:iam::1234567890:role/lacework_iam_example_role"
+    external_id = "12345"
+  }
+}
+
+resource "lacework_integration_ecr" "access_key" {
+  name            = "ECR using Access Keys"
+  registry_domain = "YourAWSAccount.dkr.ecr.YourRegion.amazonaws.com"
+  credentials {
+    access_key_id     = "AWS123abcAccessKeyID"
+    secret_access_key = "AWS123abc123abcSecretAccessKey0000000000"
+  }
+
+  limit_by_tag   = "dev*"
+  limit_by_label = "*label"
+  limit_by_repos = "my-repo,other-repo"
+  limit_num_imgs = 10
 }