Allow maintainers to toggle write permission for public deploy keys

app/controllers/projects/deploy_keys_controller.rb

app/policies/deploy_keys_project_policy.rb

app/presenters/projects/settings/deploy_keys_presenter.rb

app/serializers/deploy_key_entity.rb

app/views/shared/deploy_keys/_form.html.haml

.../unreleased/23079-write-permission-global-deploy-keys.yml

lib/api/deploy_keys.rb

spec/controllers/projects/deploy_keys_controller_spec.rb

spec/features/projects/settings/repository_settings_spec.rb

spec/policies/deploy_keys_project_policy_spec.rb

spec/requests/api/deploy_keys_spec.rb

spec/serializers/deploy_key_entity_spec.rb