Allow experimental features to be toggled individually

[brunoapimentel]

Currently, Cachi2 allows the user to specify the dev-package-managers flag to enable the usage of experimental package managers. This, however, brings the problem that all experimental package managers are enabled when this flag is set, which might confuse users into thinking a specific package manager is not experimental.

We need to implement a mechanism to allow experimental features to be toggled individually. One suggestion is to prefix x to a package manager's name to indicate it is still experimental:

cachi2 fetch-deps --source ./my-repo '{"type": "x-dnf"}'