Add resource locking

What does this MR do and why?

Add resource locking.

In order to handle race condition situation and not to let customer assign seats more than bought, we have added with_lock mechanism.

Screenshots or screen recordings

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n/a

How to set up and validate locally

1. Check out the branch
2. Create a new root group namespace: "root-group"
3. Setup some seed records and test the race condition

namespace = Namespace.last
add_on = GitlabSubscriptions::AddOn.find_or_create_by!(name: "code_suggestions") {|e| e.description = "Test"}
add_on_purchase = GitlabSubscriptions::AddOnPurchase.create!(
  add_on: add_on, namespace: namespace, expires_on: 1.month.from_now, quantity: 1, purchase_xid: 'A-S0001'
)

add_on_purchase.quantity # returns 1
add_on_purchase.assigned_users.count # returns 0

users = User.last(5) # get 5 users to add as group_member and assign on 1 available seats

users.map do |user|
  namespace.add_developer(user)
  Thread.new do
    GitlabSubscriptions::UserAddOnAssignments::CreateService.new(
      add_on_purchase: add_on_purchase,
      user: user
    ).execute
  end
end.each(&:join)

add_on_purchase.assigned_users.count # returns 1

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 #415584