[FEATURE] Add support for passing headers when writing tests

[DarshitChanpura]

Is your feature request related to a problem?

This feature requests modification of the test framework currently implemented to allow usage of headers when writing tests. This would allow for usage of credentials other than admin when writing tests. This would grant more flexibility especially for security related APIs.

What solution would you like?

• ability to use credentials other than admin when writing tests for APIs.

  - synopsis: Change password.
    path: /_plugins/_security/api/account
    method: PUT
    headers:
      authorization: "Basic <base64-encoded-string>"   <----
    request_body:
      payload:
        current_password: "myWeakPassword123!"
        password: "myStrongPassword123!"
    response:
      status: 200

[nhtruong]

Since the username and password can be included in the headers for authorization, we should add the ability to specify the headers in test stories. It will be needed in the future and will also resolve this.

@dblock what do you think?

[dblock]

@nhtruong Good idea, any headers would overwrite the ones we send otherwise.

[dblock]

I'll take this one.

[dblock]

#461