Turn cargo-mobile into a library

[lucasfernog]

We're interested in using this awesome project as a dependency on the tauri CLI as part of our upcoming mobile support. Currently I think I could run the cargo apple and cargo android commands using exec and Input, but ideally I'd like to have more control like using a different configuration file name, doing something before build starts, using a different project template etc. Would you accept a PR turning this into a library, exposing some existing modules and types? Hopefully the Tauri team can contribute back to this project in the future too, we expect a lot of users to benefit from this.