feat: add tfe_agent_token ephemeral resource

[uturunku1]

Description

Allow users to create an ephemeral resource for tfe_agent_token, so that agent pool tokens are not stored in the Terraform state file and can be handled securely and temporarily during Terraform operations.

Remember to:

• Update the Change Log
• Update the Documentation

Testing plan

1. Create a terraform configuration that has child module
2. In that module, try out a config similar to this:

resource "tfe_agent_pool" "foobar" {
  name         = "agent-pool-test"
  organization = "hashicorp"
}

ephemeral "tfe_agent_token" "this" {
  agent_pool_id = tfe_agent_pool.foobar.id
  description   = "my description"
}

output "my-agent-token" {
  value       = ephemeral.tfe_agent_token.this.token
  description = "Token for tfe agent."
  ephemeral   = true
}

Then init and apply the configuration.
The state file should not include the ephemeral resource.

External links

Include any links here that might be helpful for people reviewing your PR. If there are none, feel free to delete this section.

• go-tfe documentation
• API documentation
• Related PR

Output from acceptance tests

Please run applicable acceptance tests locally and include the output here. See testing.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

...