Sunshine: built in game/application scanner

### Is there an existing issue for this item?

- [x] I have searched the existing issues

### Repositories

LizardByte/Sunshine

### Languages/Skills/Technologies

C++, Windows, Linux, macOS

### Description

Sunshine desperately needs a built in game/app scanner.

We can initially start here:

- Epic

> To find the Epic Games app ID, go to C: \ Program Files \ Epic Games \ "Name of the Game" \.egstore. Then open the MANCPN file in notepad. The game ID will be in the “AppName” field.

https://enestech.zendesk.com/hc/en-gb/articles/360010884420-Where-to-find-Steam-Origin-Battle-net-Riot-Games-Epic-Games-app-ID

the Surviving Mars example (from the docs) of that file is
```json
{
"FormatVersion": 0,
"CatalogNamespace": "d759128018124dcabb1fbee9bb28e178",
"CatalogItemId": "20729b9176c241f0b617c5723e70ec2d",
"AppName": "Ovenbird"
}
```

Both the CatalogNamespace and CatalogItemId are required to generate the URI.

- Steam

https://stackoverflow.com/a/39435219/11214013

### Estimated Effort

effort:Large

### Priority

priority:High

### Target Milestone

3-6 months

### Dependencies

_No response_

No source file, test, or entry point is named. Start by reviewing the Sunshine repository to locate application discovery and URI-generation code, then define the scanner's expected behavior for Epic and Steam across Windows, Linux, and macOS. Done means detecting supported applications and generating the required launch URIs, including Epic's CatalogNamespace and CatalogItemId.

Written by the indexing model from the issue text.