Support for `ring.swagger.json-schema/describe`

Hello,

In my project I have used `ring.swagger.json-schema/describe` to annotate some types

```clojure
(require '[ring.swagger.json-schema :refer [describe]] '[schema.core :as s])
(s/defschema Foo (describe s/Str "Not very bar"))
```
However the `schema-tools.swagger.core/transform` failed on this
```clojure
(require '[schema-tools.swagger.core])
(schema-tools.swagger.core/transform Foo nil)
;; Execution error (ExceptionInfo) at schema-tools.swagger.core/not-supported! (core.cljc:56).
;; don't know how to convert ring.swagger.json_schema.FieldSchema@8d6443 into a Swagger Schema.
```
I have had to do the following
```clojure
(import ring.swagger.json_schema.FieldSchema)
(extend-protocol schema-tools.swagger.core/SwaggerSchema
FieldSchema
(-transform [this opts]
(schema-tools.swagger.core/transform (.schema this) opts)))
```
to get it working.

I am not sure if there should be support for this module in schema-tools, but at least it gave me some headache while trying to move from compojure-api to Reitit, so maybe it should be supported somewhere in the toolchain if I am not alone using that function !

Start by reproducing the failure at schema-tools.swagger.core/transform with ring.swagger.json-schema/describe and inspect the FieldSchema value from ring.swagger.json_schema. Read the SwaggerSchema protocol and the not-supported! path in schema-tools.swagger.core. Done means FieldSchema is transformed without the consumer defining an extension protocol.

Written by the indexing model from the issue text.