Add custom flag support

[Tomaszal]

Allows users to specify persistent flags via code-flags.conf file. Implementation taken from Spotify Flatpak.

This is an alternative to #373 that would allow users to fix #281 without their changes being reset every update. It can also potentially be useful for other things, but I imagine this would be the primary use for now.

[flathubbot]

Started test build 87601

[flathubbot]

Build 87601 failed

[Tomaszal]

Build failed with the following error:

{
    "errors": [
        "finish-args-unnecessary-xdg-config-access"
    ],
    "warnings": [
        "flathub-json-redundant-only-arches"
    ]
}

I tried to build it locally and I get the same error currently even without this change. The way code-flags.conf is accessed is no different to flatpak-vscode-first-run is accessed, so I don't think this change causes this build error.