Allow linking to add-ons through Heroku SSO

### 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?

Heroku Add-ons

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

## Problem description

Many Heroku Add-ons provide a GUI for viewing or filtering data. Many of the add-ons available on Heroku's platform have query parameters or other options in the URL that can specify things like what date range we're looking at, what term we're searching for, etc.

Recently, one of our Add-ons went away from Heroku: Logentries, now known as InsightOps. We followed them off of Heroku's Add-on infrastructure and to their standalone product, and it is truly game changing for us to be able to link to a specific state of our Add-on's GUI.

For instance, we want to point our operations team to a specific ID of some event that happened in our logs.

With Heroku's SSO not allowing us to forward through to an Add-ons URL, the way it works today, we have to ask them to:
1. go to Heroku's homepage or perhaps dashboard.heroku.com
2. login via Heroku's multi-factor auth SSO
3. find and click into a specific Heroku app (whichever one has the add-on)
4. find that Add-on in the list of Add-ons in that Heroku app
5. once in that Add-on's GUI, enter in a search term
6. also remember to define the date range of the log data over which to search
7. also make sure no other filtering is active in this GUI

If, however, we could link to that add-on directly, we could change this process to:
1. follow a link to the add-on that authenticates through Heroku SSO
2. login via Heroku's multi-factor auth SSO, and it will forward you to the exact URL that has all the filtering and search terms already setup

This would represent a huge reduction in the amount of effort to access Add-ons.

## Example of workflow change

Right now, when we're on an add-on, we see a URL that looks something like this:

`https://thiscompanyaddon.com/main/search?startdate=2023-08-01&enddate=2023-08-15&term=foobar`

If I copy/paste this link to a teammate, or generate it from within some admin tools we build for our operational team, they can visit it, but unless they already logged in via Heroku recently, that URL will not work. It will ask them to login via the `thiscompanyaddon.com` URL's accounts... but we shouldn't do that, we should login via Heroku's SSO.

If, instead, I could build a link to a URL somewhat like this...

`https://addons-sso.heroku.com/apps/auth-and-redirect?destination=https://thiscompanyaddon.com/main/search?startdate=2023-08-01&enddate=2023-08-15&term=foobar`

...and then that would have me authenticate through Heroku if needed, and then result in me reaching `https://thiscompanyaddon.com/main/search?startdate=2023-08-01&enddate=2023-08-15&term=foobar` , we would be having a massive savings in time and effort and cognitive load about what we have to do to access UIs like this.

## Some pieces already exist

Right now, I see that the links here...


...go to URLs like: https://addons-sso.heroku.com/apps/332abc-abcd-abcd-def1-2345679898/addons/4819ade-1249a-dfa-134-afd38419

Unfortunately, I don't know how to generate a URL like that. But that URL takes us to one of the add-ons. I just want to be able to supply my own URL that the addons-sso.heroku.com system will forward me to once the authentication is done.

## Why is this important? Continued spending on Heroku's platform

I hope this is possible, because we have experience this change now that we're using InsightOps directly. And because we're using it directly after it moved away from Heroku, we are paying Heroku less money. And this direct linking mechanism for filtering data is something we really need on certain types of apps going forward.

So if we could link to Add-on GUIs like this in the future, it will keep us spending the same amount with Heroku or more. But if it doesn't change, we will likely stop using Heroku for certain types of add-ons, because the operational overhead we have to do just to link to a certain part of an add-ons GUI is so heavyweight.

We have been using Add-ons for over a decade now, but now that we see how useful direct links are, I think it's going to becoming increasingly important.

Thanks!