[rust-server] Drop file support (#547)

* [rust-server] drop 'file' support

In swagger v2, we had 'binary', 'byte', and 'file'. OpenAPI v3 only has
the former two. This commit drops the old 'file' handling. This has the
side-effect of removing a half-complete implementation of form parameter handling.

This removes the ability to send files as streams, so will make life
harder for those wishing to send large files without running out of
memory.

* Remove all remaining uses of `hasFile`

samples/server/petstore/rust-server/api/openapi.yaml

@@ -657,6 +657,19 @@ paths:
           - -1.2
           format: double
           type: number
+      requestBody:
+        content:
+          application/x-www-form-urlencoded:
+            schema:
+              properties:
+                enum_form_string:
+                  default: -efg
+                  description: Form parameter enum test (string)
+                  enum:
+                  - _abc
+                  - -efg
+                  - (xyz)
+                  type: string
       responses:
         400:
           content: {}
@@ -738,6 +751,10 @@ paths:
                   description: None
                   pattern: ^[A-Z].*
                   type: string
+                byte:
+                  description: None
+                  format: byte
+                  type: string
                 binary:
                   description: None
                   format: binary
@@ -760,6 +777,7 @@ paths:
                   description: None
                   type: string
               required:
+              - byte
               - double
               - number
               - pattern_without_delimiter
@@ -1247,6 +1265,13 @@ components:
               $ref: '#/components/schemas/ReadOnlyFirst'
             type: array
           type: array
+        array_of_enum:
+          items:
+            enum:
+            - UPPER
+            - lower
+            type: string
+          type: array
       type: object
     OuterComposite:
       example:
@@ -1297,6 +1322,9 @@ components:
           format: byte
           pattern: ^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$
           type: string
+        binary:
+          format: binary
+          type: string
         date:
           format: date
           type: string
@@ -1331,6 +1359,15 @@ components:
             - crab
             type: string
           type: array
+        array_array_enum:
+          items:
+            items:
+              enum:
+              - Cat
+              - Dog
+              type: string
+            type: array
+          type: array
       type: object
     OuterString:
       type: string
@@ -1370,6 +1407,15 @@ components:
               type: string
             type: object
           type: object
+        map_map_of_enum:
+          additionalProperties:
+            additionalProperties:
+              enum:
+              - UPPER
+              - lower
+              type: string
+            type: object
+          type: object
         map_of_enum_string:
           additionalProperties:
             enum: