Update TagKeys to support purpose and purposeData

[modular-magician]

This PR adds the additional properties purpose and purposeData to the google_tags_tag_key resource. These additional properties enable the ability to deploy Secure Tags, which can then be used by Firewall Policies.

Since both purpose and purpose_data cannot be modified once set, both resources have input: true set in their definitions. Additionally, the ignore_read override is set on purpose_data. This is because although purpose_data can accept input such as the following:
- {network = "[project-id]/[vpc-name]}
- {network = "[project-id]/[vpc-id]"}
- {network = "https://www.googleapis.com/compute/v1/projects/[project-id]/global/networks/[vpc-id]"}

The API's response is always stored in the state as {network = "https://www.googleapis.com/compute/v1/projects/[project-id]/global/networks/[vpc-id]"} (also known as selfLinkWithId). As a result, persistent diffs would be generated without ignore_read, which stores the user input instead of the API's response. The API does validate that the referenced network (VPC) exists and is accessible, so using the ignore_read override will not result in a corrupted or broken state.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

tags: Added `purpose` and `purpose_data` properties to `google_tags_tag_key`

Derived from GoogleCloudPlatform/magic-modules#6593

[github-actions]

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