feat(alerting): Add subscriptions alerts API

[vankiru]

Context

The PR implements missing api for subscription alerts. It relies on the implementation of Nested Commands introduced in #366.

Related PR: getlago/lago-api#4925

Description

The wallet alerts API looks like this 👇

# GET /subscriptions/:subscription_id/alerts
client.subscriptions.alerts.get_all(subscription_id)
client.subscriptions.alerts.get_all(subscription_id, options={"page": 1, "per_page": 100})

# POST /subscriptions/:subscription_id/alerts
client.subscriptions.alerts.create(subscription_id, params)

# GET /subscriptions/:subscription_id/alerts/:alert_code
client.subscriptions.alerts.get(subscription_id, alert_code)

# PATCH /subscriptions/:subscription_id/alerts/:alert_code
client.subscriptions.alerts.update(subscription_id, alert_code, params)

# DELETE /subscriptions/:subscription_id/alerts/:alert_code
client.subscriptions.alerts.destroy(subscription_id, alert_code)

[annvelents]

checked all the values for Alerts for both: you and me and updated my model to include values that you have. The only difference we have now is Alert vs AlertInput

[annvelents]

cannot be null due to validation on the DB

[vankiru]

All the input classes are just called {Model} without Input suffix, so I kept the same naming for alerts as well 😊

I initially marked everything as optional in the model definitions since that's how most of the other models are set up 🤔 But I've updated the alerts model to require the fields you mentioned.

[annvelents]

cannot be null also

[annvelents]

thresholds should be provided - required by the service

[annvelents]

value cannot be null 🙈

[annvelents]

and this one IS optional 😅