See template-server/template-server.md for design, and ttv-wizard.mvp
for (first) application.

Just have a single Expand call that expands twice, once to `io.Discard` and
once to the real writer.

This should makes it immediately and directly usable.

Co-authored-by: Jonathan Rosenberg <96974219+Jonathan-Rosenberg@users.noreply.github.com>

* Move `template_location` to the URL path.  This makes the OpenAPI
  interface cleaner: the URL query now contains just the parameters
  of the template.
* [CR] Make the `spark.conf.tt` template the finished required form
  including `lakefs_url` in it.
* Fix **bug** in OpenAPI golang code generation (on both client and
  server sides) with freeform query params in the specification: it
  does not understand `expand: true`.  This is documented as how to
  do it, generates correct URLs in online OpenAPI editors, but code
  generated is incorrect (seems to think the dummy params object is
  actually where it will place the parameters, when in fact it does
  not.