Immutable `restructure` interface

Nobody has claimed this yet.

Instead of a big `restructure` multimethod that you need to manage extensions for, perhaps we can encourage more mix-and-match approaches that start to look like the kind of centralized config you'd expect from reitit.

One of the motivations is having full control and certainty over the restructure options for security purposes.

Since compojure-api is macro-based, the main obstacle here is forwarding the immutable config to the macros. This can be accomplished by a macro-generating macro.

e.g.,
```clojure
(ns my-bare-bones-compojure-api
"A version of compojure-api that only supports :tags."
(:require [compojure.api.immutable :as im]
[compojure.api.meta.tags]
[clojure.set :as set]))

(def ^:private options
{:restructure {:tags compojure.api.meta.tags/extension}})

(im/create-compojure-api `options)
=>
(defmacro create-compojure-api [options]
(assert (and (seq? options)
(= 2 (count options))
(= 'quote (first options))
(qualified-symbol? (second options)))
"Options must be a quoted qualified symbol whose var contains your configuration, like: (load-api `options)")
`(let [options# ~options]
(defmacro ~'GET {:style/indent 2 :arglists '([& ~'args])} [& args#] (GET options# args#))
(defmacro ~'ANY {:style/indent 2 :arglists '([& ~'args])} [& args#] (ANY options# args#))
(defmacro ~'PATCH {:style/indent 2 :arglists '([& ~'args])} [& args#] (PATCH options# args#))
(defmacro ~'DELETE {:style/indent 2 :arglists '([& ~'args])} [& args#] (DELETE options# args#))
(defmacro ~'POST {:style/indent 2 :arglists '([& ~'args])} [& args#] (POST options# args#))
(defmacro ~'PUT {:style/indent 2 :arglists '([& ~'args])} [& args#] (PUT options# args#))
(defmacro ~'context
"Like compojure.api.core/context, except the binding vector must be empty and
no binding-style options are allowed. This is to prevent the passed routes
from being reinitialized on every request."
{:style/indent 2 :arglists '~'([path arg & args])}
[path# arg# & args#]
(context options# path# arg# args#))
(defn ~'routes
"Create a Ring handler by combining several handlers into one."
{:style/indent 2 :arglists '~'([& handlers])}
[& handlers#]
(routes options# handlers#))
(defmacro ~'middleware
"Wraps routes with given middlewares using thread-first macro.
Note that middlewares will be executed even if routes in body
do not match the request uri. Be careful with middlewares that
have side-effects."
{:style/indent 1 :arglists '~'([middleware & body-exprs])}
[middleware# & body-exprs#]
(middleware options# middleware# body-exprs#))
(defmacro ~'undocumented
"Routes without route-documentation. Can be used to wrap routes,
not satisfying compojure.api.routes/Routing -protocol."
{:arglists '~'([& handlers])}
[& handlers#]
(undocumented options# handlers#))))
```

Start with the proposed create-compojure-api macro and its generated GET, ANY, PATCH, DELETE, POST, PUT, context, routes, middleware, and undocumented entry points. Review how the immutable options map is forwarded to these macros and how restructure extensions are selected. Done means an immutable, restricted configuration can generate the intended API without relying on the large extensible restructure multimethod.

Written by the indexing model from the issue text.