xtask: Builds struct

[gabriele-0201]

In #221, the xtask/src/build.rs file contains a single function. I foresee the creation of a 'Builds' struct that will store all the necessary information for building the binaries, such as debug or release flags. Moreover, it would be advantageous to incorporate the option of selectively building specific binaries, rather than only having the choice of building all or skipping all.