Add injecting header parameters

[guillaumeserale]

I've added the generation of http headers in src/Refitter.Core/ParameterExtractor.cs.
This can be disabled by setting RefitGeneratorSettings.GenerateOperationHeaders to false.

[christianhelle]

@guillaumeserale Thank you for this contribution

If it's not too much trouble, can you please provide some example OpenAPI specifications with header parameters in a request? Like how it is done in #7

[guillaumeserale]

@christianhelle Sure!
There's already an example in the unit tests:
I've trimmed it down to only the delete operation that uses a header:

info:
  title: Swagger Petstore
  description: |-
    This is a sample Pet Store Server based on the OpenAPI 3.0 specification.  You can find out more about
    Swagger at [http://swagger.io](http://swagger.io). In the third iteration of the pet store, we've switched to the design first approach!
    You can now help us improve the API whether it's by making changes to the definition itself or to the code.
    That way, with time, we can improve the API in general, and expose some of the new features in OAS3.

    Some useful links:
    - [The Pet Store repository](https://github.com/swagger-api/swagger-petstore)
    - [The source API definition for the Pet Store](https://github.com/swagger-api/swagger-petstore/blob/master/src/main/resources/openapi.yaml)
  termsOfService: 'http://swagger.io/terms/'
  contact:
    email: apiteam@swagger.io
  license:
    name: Apache 2.0
    url: 'http://www.apache.org/licenses/LICENSE-2.0.html'
  version: 1.0.5
externalDocs:
  description: Find out more about Swagger
  url: 'http://swagger.io'
tags:
  - name: pet
    description: Everything about your Pets
    externalDocs:
      description: Find out more
      url: 'http://swagger.io' 
paths:  
  '/pet/{petId}':
    delete:
      tags:
        - pet
      summary: Deletes a pet
      description: ''
      operationId: deletePet
      parameters:
        - name: api_key
          in: header
          description: ''
          required: false
          schema:
            type: string
        - name: petId
          in: path
          description: Pet id to delete
          required: true
          schema:
            type: integer
            format: int64
      responses:
        '400':
          description: Invalid pet value
      security:
        - petstore_auth:
            - 'write:pets'
            - 'read:pets'
components:  
  securitySchemes:
    petstore_auth:
      type: oauth2
      flows:
        implicit:
          authorizationUrl: 'https://petstore3.swagger.io/oauth/authorize'
          scopes:
            'write:pets': modify pets in your account
            'read:pets': read your pets
    api_key:
      type: apiKey
      name: api_key
      in: header

[christianhelle]

This feature is now released as v0.5.1 at nuget.org

Thanks again @guillaumeserale