Introduce root endpoints' schemas

[yeralin]

Summary

Addresses #289
Adds schemas that describe request and response payloads for "root" endpoints.

Details and comments

I found couple problems while preparing this PR:

• Since we are using Marhsmallow 2.0, marshmallow.fileds.Dict is not fully supported
  If you look at the official doc for Dict field under Marshmallow 2.0: https://marshmallow.readthedocs.io/en/2.x-line/api_reference.html#marshmallow.fields.Dict
  It states: "This field is only appropriate when the structure of nested data is not known." i.e. if you use mashmallow.fields.Dict field under your schema, it will be deserialized as a simple dict object, no support for marshmallow.fields.Nested.
  However, if you look at docs for Dict in Marshmallow 3.0:
  https://marshmallow.readthedocs.io/en/stable/api_reference.html#marshmallow.fields.Dict
  You can explicitly set a nested schema for keys and values like:

fields.Dict(keys=fields.Str(), values=fields.Float())

They discussed the implementation of this feature under marshmallow-code/marshmallow#483
Because of this limitation schemas like HubsResponseSchema and CircuitResponseSchema can only be deserialized partially.

Also because of this issue, marshmallow-jsonschema package cannot generate a proper JSON schema for nested dicts, and rendered as <not serializable>:

TL;DR we need to upgrade Marshmallow version to 3.0, if we want to fully deserialize schemas like HubsResponseSchema and CircuitResponseSchema

• Deserialization of HubsResponseSchema will always fail bc @bind_schema does not allow passing many = True

The issue was raised under qiskit-terra repo: Qiskit/qiskit#3021
If we were to create a model for HubsResponseSchema using @bind_schema decorator, and then use it to deserialize hubs response payload, the deserialization will always fail with ModelValidationError. The reason is bc the payload is of the following format: [<payload>] i.e. the payload is wrapped in an array. This could be easily fixed by passing many = True while creating an instance of HubsResponseSchema(many=True), but the instance is created internally in @bind_schema decorator, and we cannot pass any arguments inside.

[diego-plan9]

Following up on HubsResponseSchema: let's decouple the issue with that particular response in order to move forward, as at this point in time there is no need for worrying about serialization and we can handle that response via other means.

Can you make HubsResponseSchema a dummy schema (with no fields), and add a small note in the class to remind us that it needs to be treated slightly differently for the time being?