Add sso-readonly provider to platform_providers

[dms1981]

Adds a new provider block - aws.sso-readonly - to allow customers to conduct data call lookups against SSO/IdentityStore in the MOJ root account.

Also adds an example showing a data call based off of the Hashicorp documentation.

[github-actions]

TFSEC Scan Success

Show Output

```hcl

---

TFSEC will check the following folders:
terraform/environments/corporate-staff-rostering

---

Running TFSEC in terraform/environments/corporate-staff-rostering
Excluding the following checks: AWS095

======================================================
tfsec is joining the Trivy family

tfsec will continue to remain available
for the time being, although our engineering
attention will be directed at Trivy going forward.

You can read more here:
aquasecurity/tfsec#1994

timings
──────────────────────────────────────────
disk i/o 1.667455ms
parsing 257.904758ms
adaptation 150.655µs
checks 10.194379ms
total 269.917247ms

counts
──────────────────────────────────────────
modules downloaded 0
modules processed 5
blocks processed 291
files read 76

results
──────────────────────────────────────────
passed 1
ignored 1
critical 0
high 0
medium 0
low 0

No problems detected!

tfsec_exitcode=0

</details> #### `Checkov Scan` Failed
<details><summary>Show Output</summary>

```hcl

*****************************

Checkov will check the following folders:
terraform/environments/corporate-staff-rostering

*****************************

Running Checkov in terraform/environments/corporate-staff-rostering
2024-02-12 13:05:52,707 [MainThread  ] [WARNI]  Failed to download module github.com/ministryofjustice/modernisation-platform-terraform-lambda-function:None (for external modules, the --download-external-modules flag is required)
terraform scan results:

Passed checks: 142, Failed checks: 1, Skipped checks: 26

Check: CKV_TF_1: "Ensure Terraform module sources use a commit hash"
	FAILED for resource: ad-clean-up-lambda
	File: /lambda.tf:7-33
	Guide: https://docs.prismacloud.io/en/enterprise-edition/policy-reference/supply-chain-policies/terraform-policies/ensure-terraform-module-sources-use-git-url-with-commit-hash-revision

		7  | module "ad-clean-up-lambda" {
		8  |   source = "github.com/ministryofjustice/modernisation-platform-terraform-lambda-function" # ref for V3.1
		9  |   count  = local.environment == "test" ? 1 : 0                                             # temporary whilst on-going work
		10 | 
		11 | 
		12 |   application_name = local.lambda_ad_object_cleanup.function_name
		13 |   function_name    = local.lambda_ad_object_cleanup.function_name
		14 |   description      = "Lambda to remove corresponding computer object from Active Directory upon server termination"
		15 |   package_type     = "Zip"
		16 |   filename         = data.archive_file.ad-cleanup-lambda.output_path
		17 |   source_code_hash = data.archive_file.ad-cleanup-lambda.output_base64sha256
		18 |   handler          = "lambda_function.lambda_handler"
		19 |   runtime          = "python3.8"
		20 | 
		21 |   create_role = false
		22 |   lambda_role = aws_iam_role.lambda-ad-role[count.index].arn
		23 | 
		24 |   vpc_subnet_ids         = tolist(data.aws_subnets.shared-private.ids)
		25 |   vpc_security_group_ids = [module.baseline.security_groups["domain"].id]
		26 | 
		27 |   tags = merge(
		28 |     local.tags,
		29 |     {
		30 |       Name = "ad-object-clean-up-lambda"
		31 |     },
		32 |   )
		33 | }


checkov_exitcode=1

CTFLint Scan Failed

Show Output

*****************************

Setting default tflint config...
Running tflint --init...
Installing "terraform" plugin...
Installed "terraform" (source: github.com/terraform-linters/tflint-ruleset-terraform, version: 0.5.0)
tflint will check the following folders:
terraform/environments/corporate-staff-rostering

*****************************

Running tflint in terraform/environments/corporate-staff-rostering
Excluding the following checks: terraform_unused_declarations
2 issue(s) found:

Warning: Module source "github.com/ministryofjustice/modernisation-platform-terraform-lambda-function" is not pinned (terraform_module_pinned_source)

  on terraform/environments/corporate-staff-rostering/lambda.tf line 8:
   8:   source = "github.com/ministryofjustice/modernisation-platform-terraform-lambda-function" # ref for V3.1

Reference: https://github.com/terraform-linters/tflint-ruleset-terraform/blob/v0.5.0/docs/rules/terraform_module_pinned_source.md

Warning: Missing version constraint for provider "archive" in `required_providers` (terraform_required_providers)

  on terraform/environments/corporate-staff-rostering/lambda.tf line 35:
  35: data "archive_file" "ad-cleanup-lambda" {

Reference: https://github.com/terraform-linters/tflint-ruleset-terraform/blob/v0.5.0/docs/rules/terraform_required_providers.md

tflint_exitcode=2

Trivy Scan

Show Output