Add jitsu_key to EE::ProjectType

What does this MR do and why?

• Adds a new field jitsu_key to the EE::ProjectType graphql type.
• It delegates the method to project.project_setting

How to set up and validate locally

• With a valid ultimate licence
• Create a project, and update its jitsu_key. The value it unimportant in this MR:

project = Project.last
project.project_setting.update!(jitsu_key: 'testtest')

• Enable the feature flag

Feature.enable(:cube_api_proxy)

• As a project developer (or higher), make the following graphql query:

query {
  project(fullPath: "FULL PATH TO PROJECT") {
    jitsuKey
  }
}

• Validate that the value is returned.
• Also validate that with the feature flag disabled, the wrong licence or a user of insufficient perrmisions is returned null in the JSON response.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #381332 (closed)