new: Add linode_placement_group data source

[yec-akamai]

📝 Description

Adding the data source linode_placement_group to read detailed information about a Linode placement group.

✔️ How to Test

make PKG_NAME="linode/placementgroup" int-test

Manual test:

1. In a sandbox environment, i.e. dx-devenv, run the following script to read the linode_placement_group data source.

data "linode_placement_group" "test" {
    id = {your-pg-id}
}

output "result" {
    value = data.linode_placement_group.test
}

2. Observe the detailed information of the above placement group is outputed.

[lgarber-akamai]

E2E tests are all passing locally for me, nice work!

[jriddle-linode]

LGTM, works locally.