Coder Tasks requires us to create a workspace, but we want to be able to
return a `codersdk.Task` instead of a `codersdk.Workspace`. This
requires untangling `createWorkspace` from directly writing to
`http.ResponseWriter`.
`BuildError` response from `wsbuilder` does not support rich errors from validation. Changed this to use the `Validations` block of codersdk responses to return all errors for invalid parameters.