Fixed Firefox customization issue and added support for addons in my-resources

[Skileau]

Firefox customization now works as announced in PR #107. 🦊
Also, the user is now able to add his favourites addons to each new container with my-resources by two ways.

With a list

It is possible to create a list in ~/.exegol/my-resources/setup/firefox/addons.txt with the links in Mozilla's addon shop :

The last versions of the addons will then be downloaded and installed automatically.

With xpi files

It is also possible to directly add .xpi files in the folder ~/.exegol/my-resources/setup/firefox/addons :

The addons will then be installed from local .xpi files.

Note : both way can be used simultaneously by just creating the addons.txt and the addons folder.

[ShutdownRepo]

test failure is unrelated to the PR and is due to runner's network instability
pipeline is ok, let's wait for @Dramelac's review

[Dramelac]

@ShutdownRepo good for merge ?

[ShutdownRepo]

@Skileau can you confirm custom cert trusting will be handled in another PR?

[Skileau]

@Skileau can you confirm custom cert trusting will be handled in another PR?

Yes, you can merge, I will create a new PR with cert trusting support in the future.
We will define together the structure of the code for this new PR.