Configuring spend allocation for future periods so we can allocate the entire quota and not just available quota

### Problem Statement

Spend allocations are renewed monthly, so a new plan can be set and configured every month, or an existing configuration can last several months.

Currently, it is only possible to allocate the available quota instead of the entire quota, so the users can only configure the remaining amount of a month, instead of planning the upcoming months.

Imagine the following scenario: a project joins Sentry misconfigured, spending the whole company quota. By that time, the team discovers the spend allocation feature and wants to configure it for preventing this scenario to happen in the following months.

The team won't be able to configure it since there will be zero errors available, and it will have to wait until the turn of the next month to apply the configurations.

The experience could be even worse if the spend allocation is an automated process that runs on random day.

### Solution Brainstorm

Users should be able to configure spend allocation for the upcoming months.

This could be as simple as allowing a single configuration that will be valid for every following month, taking place on the current month only if there is an available quota; or it could be more complex, with two or more configurations based on range periods.

### Product Area

Other

┆Issue is synchronized with this [Jira Improvement](https://getsentry.atlassian.net/browse/FEEDBACK-1888) by [Unito](https://www.unito.io)