echoprovider: Add a new test-only v6 provider that echoes ephemeral provider data

[austinvalle]

The changes in this PR can be seen in use over at: hashicorp/terraform-provider-corner#283

This PR contains an implementation of a v6 Terraform provider that can be used to test ephemeral resources. As ephemeral data is not stored in Terraform artifacts (plan or state), that means that the terraform-plugin-testing Go module cannot help provider developers test that data directly.

The echo provider utilizes the ephemeral nature of provider configurations as a "passthrough" for data to be echoed later by the single echo managed resource type:

ephemeral "examplecloud_thing" "one" {
  name = "John Doe"
}

provider "echo" {
  # Provide the ephemeral data we want to run test assertions against
  data = ephemeral.examplecloud_thing.one
}

# The ephemeral data will be echoed into state
resource "echo" "test_one" {}

output "name_out" {
  # Will output "John Doe"!
  value = echo.test_one.data.name
}	

A more detailed example, along with accompanying state checks, can be found in the PRs website documentation: https://github.com/hashicorp/terraform-plugin-testing/blob/av/ephemeral-values/website/docs/plugin/testing/acceptance-tests/ephemeral-resources.mdx#testing-ephemeral-data-with-echo-provider

---

Eventually, other methods of inspecting/consuming ephemeral data may be introduced that might warrant revisiting this provider, such as:

• Terraform core implementing their own "debug" utility for practitioners inspecting ephemeral data
• Write-only attributes in managed resources

But for now, ephemeral data => provider config => managed resource state is essentially the only reasonable option for CI testing. (outside of using the remote system, which is encouraged in the docs)

Notes

• This PR also contains a new version constraint for the upcoming Terraform v1.10.0 release and an implementation of EphemeralResourceServer RPCs as mentioned in build(deps): Bump github.com/hashicorp/terraform-plugin-go from 0.24.0 to 0.25.0 #382 (comment)
• echoprovider is implemented as a v6 provider because it's intended use is with ephemeral data, which exists only in Terraform v1.10.0+