[feat] Project reward claims stock

[subiabre]

In #36 were added the models necessary to store project rewards and user claims for rewards.

This PR implements the business logic necessary to make the reward claims work when they have limited units, making each reward claim substract from the available units in the reward.

[subiabre]

@davidbeig can you give this a good look for a review?

I'm concerned about solidifying the custom providers / processors to specify custom behavior, inject the service / business logic on resources. I tried to build this feature leaning as much into the logic model as possible and leaving reusable pieces for the future. The gist of it is that the generic, default processing flow by ApiPlatform for entities has been encapsulated in a reusable, re-injectable EntityStateProcessor, which in turn gets called by custom state processors for resources after these map the resource to an entity and after applying custom service logic to the entity object under certain their considered conditions.

This means, ultimately, that Rewards have a RewardStateProcessor which sets the available units from the total units on new rewards, and RewardClaims have a RewardClaimStateProcessor that substracts from the Reward on new reward claims.