Refactor: Model base API improvements

[sardavend]

What does this PR do?

Improve the library interface to a new Resource/Model based API by leveraging the ruby meta-programming capabilities, this way we remove the hardcoded dependencies attributes of any Fintoc's resources and also abstract away the Client object to be implicitly called by the resources/models instead of using it directly by the library users

Details

Quick start usage with the current interface

client = Fintoc::Client.new('sk_test_9c8d8CeyBTx1VcJzuDgpm4H-bywJCeSx')
link = client.get_link('6n12zLmai3lLE9Dq_token_gvEJi8FrBge4fb3cz7Wp856W')
account = link.find(type: 'checking_account')

Quick start usage with the improved interface

Fintoc.api_key = 'sk_test_9c8d8CeyBTx1VcJzuDgpm4H-bywJCeSx'
link = Fintoc::Link.find('6n12zLmai3lLE9Dq_token_gvEJi8FrBge4fb3cz7Wp856W')
account = link.accounts.find(type: 'checking_account')

Get the las 30 movements with the current interface

movements = account.get_movements
movements = account.get_movements(since: '2020-08-15')

Get the las 30 movements with the improved interface

movements = account.movements
movements = account.movements(since: '2020-08-15')