Implement selectable dependency

[pablomendezroyo]

Describe the new manifest feature you'd like

Similar to the manifest dependencies feature, add an optional selectableDependencies which contains an array of strings where the user will have to choose some of them to be installed together with the package to be installed.

The dependencies must be in the format package.dnp.dappnode.eth:0.1.0

Describe alternatives you've considered

NA

Additional context

This feature would allow users to easy install a CL package with the dependency web3signer and the selectableDependency of the EL.

[pablomendezroyo]

@dsimog01 this feature would be cool for staker pkgs where there are a lot of nested dependencies