Add Typescript HTTP method definition for LINK and UNLINK

[marlonbarcarol]

Versions of axios below 19.0 used to have the AxiosRequestConfig.method declared as a string, with the new version, methods are now declared as:

export type Method =
  | 'get' | 'GET'
  | 'delete' | 'DELETE'
  | 'head' | 'HEAD'
  | 'options' | 'OPTIONS'
  | 'post' | 'POST'
  | 'put' | 'PUT'
  | 'patch' | 'PATCH'

Since the new version typescript has been generating the errors due to the lack of the methods link and unlink:
Type '"link"' is not assignable to type 'Method'.
Type '"unlink"' is not assignable to type 'Method'.