Create a struct to manage runtime metadata files

[Angelmmiguel]

The purpose of this struct is to act as a "Runtime Metadata Manager". The main tasks of this struct will be:

• Load metadata files for new runtimes from the local filesystem or a remote one
• Download related resources and store them using the runtime::Data struct. We will need to add new methods to this one
• Validate the different files using the associated checksums

This can be split in multiple structs / modules if required.