Add utility extension point for supporting mods like OptiFine

[OptiFine](https://www.optifine.net/home) is one of the most popular mods for Minecraft as it's able to boost performance and has many parameters for tuning it further so it can run on less powerful systems.

I've been able to copy OptiFine.jar into `~/.var/app/com.mojang.Minecraft/.minecraft` and then run the following command to guarantee OptiFine can find the Minecraft home folder:

$ cp OptiFine_1.14.4_HD_U_F4.jar ~/.var/app/com.mojang.Minecraft/.minecraft/OptiFine.jar
$ flatpak run --command=/app/jre/bin/java com.mojang.Minecraft -jar ~/.minecraft/OptiFine.jar

Could we have an extension point for utilities [similar to the Steam flatpak](https://github.com/flathub/com.valvesoftware.Steam/pull/507)?

I have a mockup of a build manifest that might be useful as a starting point, although I haven't gotten it to successfully build:

```yaml
id: com.mojang.Minecraft.Utility.optifine
branch: stable
runtime: com.mojang.Minecraft
runtime-version: stable
sdk: org.freedesktop.Sdk//19.08
build-extension: true
appstream-compose: false
build-options:
prefix: /app/utils/optifine
prepend-path: /app/utils/optifine/bin
modules:
- name: optifine
buildsystem: simple
build-commands:
- install -d /app/utils/optifine/bin
- cp optifine.jar /app/utils/optifine/OptiFine.jar
sources:
- type: file
path: OptiFine_1.14.4_HD_U_F4.jar
sha356: 66df1c3b02d62cf7149992b576e34ee0a37170f68613b4d9ab5be08c792154f2
dest-filename: OptiFine.jar
- name: scripts
sources:
- type: script
commands:
- exec /app/jre/bin/java -jar /app/utils/optifine/OptiFine.jar
dest-filename: optifine
buildsystem: simple
build-commands:
- cp optifine /app/utils/optifine/bin
``