Swap to Context everywhere and diag support

[jacobbednarz]

Updates the provider to make use of Terraform diagnostics and reusing the context.Context passed in by the provider instead of spawning a new one.

Closes #1187

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[jacobbednarz]

kicked off the full acceptance test suite for this one at https://github.com/cloudflare/terraform-provider-cloudflare/actions/runs/2256240309

[github-actions]

This functionality has been released in v3.15.0 of the Terraform Cloudflare Provider.

Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!