MQTT Client billing model

This task is to cover the billing enablement of the MQTT Client feature #1350

This first iteration being delivered includes the ability to limit a Team Type to a given number of clients. From a billing perspective, this is the 'free' allocation that is included in the base team type cost.

Unlike our runtime billing model, which is based on realtime usage, the intended model is to allow users to pre-purchase additional 'blocks' of clients.

This task is to implement this billing model. It will require:

1. provide an API and UX flow for a user to purchase additional client blocks
2. track how many clients a team has purchased
3. update the 'create client' checks to ensure usage keeps within the combined free allowance and purchased allowance

```[tasklist]
### Tasks
- [ ] https://github.com/FlowFuse/flowfuse/pull/4732
- [ ] Add per-team allowance for clients
- [ ] Enabling billing driven path to modify per-term allowance
```