The codersdk was becoming bloated with consts for the workspace
agent that made no sense to a reader. `Tailnet*` is an example
of these consts.

It feels wrong to have this as a direct option because it's so infrequently
needed by API callers. It's better to directly modify headers in the two
places that we actually use it.

`codersdk.Response` lived in `error.go`, which is wrong.

It was odd conflating the codersdk that clients should use
with functions that only the agent should use. This separates
them into two SDKs that are closely coupled, but separate.