Agent: Add protobuf field to support shared namespace

MR: Add shared_namespace field to the remote develo... (gitlab-org/cluster-integration/gitlab-agent!2346 - merged)

Description

As a user, I want specify the name of the namespace in which all the new Workspaces will be created in Kubernetes so that I can run GitLab Agent with non-clusterwide permissions.

Acceptance criteria

• A new field is added in the protobuf definition of the remote_development module of the GitLab agent.
• This new field will be on type string and there will be no default value.
• This new field will be called shared_namespace.

Implementation plan

• Update the RemoteDevelopmentCF in the agent configuration proto here.
• Regenerate the protobuf definitions by following the steps here.