Add a Heroku Postgres Eco Tier

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Postgres

### Tell us about what you're trying to solve. What challenges are you facing?

The switch to Heroku `Eco` dynos seems to be a nice compromise between free and paying $7/month for each app you have. This is especially true when the app(s) is for prototypes/staging/personal usage. For me personally, I can easily stay within the `Eco` tier limitations for the few personal apps I run for myself and am happy to spend $5 for what Heroku is providing.

However, when it comes to using a database, the pricing is not as "friendly". Since using a DB is basically a necessity, especially when using Heroku because of the ephemeral filesystem, this essentially means each app costs an additional $5. In this case, for me personally, this doesn't seem worth it. When I'm storing `100s` of rows, `< 50MBs` of data, and really only running the dynos the database is attached to for `< 15` hrs/month, it is difficult for me to justify to myself the $5/app price. It is difficult to justify because for each app I am running like this, I am barely using the DB resources I am paying for. But because it is a per app price, I end up having to buy another database plan and waste most of the available resources.

I don't know how many other people are in a similar situation as I am. But I would think there are various situations where it would be great to pool your database plan limits across multiple DBs, similar to how `Eco` dynos work. Rather than a pool of hours, you would get a pooled row limit, storage capacity, etc. This way each database would still be completely separate and conveniently attached to a specific app, but from Heroku's perspective users couldn't abuse DB usage like with the previous free tier offering. If the `Mini` plan's specs were used for an `Eco` database plan, that would work well for me. And if a particular DB got too big, I could then pay to bump it up to the "normal" tier where it has its own set of specs/resources, again similar to how Heroku has setup dyno plans.

I would love to keep using Heroku Postgres because it works well, I like that it is managed, and I don't mind paying for it. It just seems silly to me to pay for something that I am essentially barely using. I would personally rather pay a fixed cost for a plan and know that I can run multiple databases within its limits. For example, I find the idea of using DigitalOcean's Managed Postgres appealing because it doesn't cost more if I create more DBs. There are tradeoffs, but it is a very compelling option for my use case. If Heroku were to offer something like this, for me, the limits of the `Mini` plan are fine. I would be curious what others think about this. And I'm sure Heroku has data around how much of the DB limits people are actually using, so I would be curious what someone from Heroku thinks as well.

I realize this is only going to be useful for a certain group of people, like myself, who are not running anything big and are not generating any money with an app (if I was I wouldn't mind paying for that app). But I hope there are enough people that do fit this bill that it could be worthwhile for Heroku to look into and perhaps even offer!

Thanks!