Currently in services, the `contract` step is only used to define where
the contract will be called in the execution flow. Then, a `Contract`
class has to be defined with validations in it.
This patch allows the `contract` step to take a block containing
validations, attributes, etc. directly. No need to then open a
`Contract` class later in the service.
It also has a nice side effect, as it’s now easy to define multiples
contracts inside the same service. Before, we had the `class_name:`
option, but it wasn’t really useful as you had to redefine a complete
new contract class.
Now, when using a name for the contract other than `default`, a new
contract will be created automatically using the provided name.
Example:
```ruby
contract(:user) do
attribute :user_id, :integer
validates :user_id, presence: true
end
```
This will create a `UserContract` class and use it, also putting the
resulting contract in `context[:user_contract]`.
This commit adds ability to fetch a subset of site settings from the `/admin/site_settings` endpoint so that it can be used in all places where the client app needs access to a subset of the site settings.
Additionally, this commit also introduces a new service class called `UpdateSiteSetting` that encapsulates all the logic that surrounds updating a site setting so that it can be used to update site setting(s) anywhere in the backend. This service comes in handy with, for example, the controller for the flags admin config area which may need to update some site settings related to flags.
Internal topic: t/130713.
