Switch to the official OpenAPI spec

[praneetloke]

When this provider was created, Tailscale's API was documented in a GitHub repo...in a markdown file. So it was necessary to generate an OpenAPI spec from something that was easy to maintain. Now Tailscale has their own API docs site with the option to download an OpenAPI spec in JSON or YAML. We should switch to that.

[praneetloke]

Initial observations:

• Type SplitDns has an additional properties schema with the type array and also 'null' instead of using the nullable attribute.
• The error response schema types for error codes 400, 403, 404, 409, 500, 501, 502 and 504 have an example defined under the schema property instead of its sibling.
• The response schema for various endpoint paths have a description property in addition to a schema ref ($ref) which is invalid.