Add Any type for empty interface{} objects

[rw-access]

Closes https://github.com/wI2L/fizz/issues/51

Added handling for empty interface{} so that it's represented like an "any" type in OpenAPI.

From the specs: https://swagger.io/docs/specification/data-models/data-types/#any

Any Type A schema without a type matches any data type – numbers, strings, objects, and so on. {} is shorthand syntax for an arbitrary-type schema:

components:
  schemas:
    AnyValue: {}

If you want to provide a description:

components:
  schemas:
    AnyValue:
      description: Can be any value - string, number, boolean, array or object.

The above is equivalent to:

components:
  schemas:
    AnyValue:
      anyOf:
        - type: string
        - type: number
        - type: integer
        - type: boolean
        - type: array
          items: {}
        - type: object

If the null value needs to be allowed, add nullable: true:

components:
  schemas:
    AnyValue:
      nullable: true
      description: Can be any value, including `null`.

[codecov[bot]]

Codecov Report

Merging #53 (5677a88) into master (7bb66a1) will decrease coverage by 0.85%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   96.17%   95.31%   -0.86%     
==========================================
  Files           7        7              
  Lines         889      897       +8     
==========================================
  Hits          855      855              
- Misses         20       27       +7     
- Partials       14       15       +1     

Impacted Files	Coverage Δ
openapi/generator.go	93.84% <0.00%> (-0.93%)	:arrow_down:
openapi/types.go	97.18% <0.00%> (-2.82%)	:arrow_down:

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