feat(iam): add iam roles data source

[andreilapkin]

What this PR does / why we need it:
Sometimes it's more convenient to all the roles in one list rather than getting a single one by filter as it's made in data.identity_role.
Very useful in scenarios with terragrunt when you can use roles list as a dependency.

PR Checklist

• Tests added/passed.

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccSomethingV0_basic'
...
~/Documents/repos/github/terraform-provider-huaweicloud master ❯ make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccSomethingV0_basic'                                                                       10:13:14
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run=TestAccSomethingV0_basic -timeout 360m -parallel 4
testing: warning: no tests to run
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       0.865s [no tests to run]

• Documentation updated.
• Schema updated.
• CheckDeleted.

[kolt2050]

That is cool for my team

[andreilapkin]

Any thoughts on merging this?

[github-ci-robot]

This PR needs rebase.