add datasource for policy_set

[Uk1288]

Description

Add datasource for policy set. This PR is in relation to the tfe_workspace_policy_set work here https://github.com/hashicorp/terraform-provider-tfe/pull/591

Remember to:

• Update the Change Log

• Update the Documentation

Testing plan

1. Use datasource like:

data "tfe_policy_set" "readable_policy_set" {
  name = "my-policy-set"
  organization = "awesome-org"
}

External links

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• Related PR

Output from acceptance tests

Please run applicable acceptance tests locally and include the output here. See TESTS.md to learn how to run acceptance tests.

If you are an external contributor, your contribution(s) will first be reviewed before running them against the project's CI pipeline.

$ TESTARGS="-run TestAccTFEWorkspace" make testacc

...