Implement Bindle module source in application manifest

[radu-matei]

The local configuration allows defining a remote reference to a bindle as the module source:

https://github.com/fermyon/spin/blob/a09fb1f2b6f9a52dee7dbcbe54e6f9ff41a72ea0/crates/loader/src/local/config.rs#L123-L131

However, that is currently not implemented:

https://github.com/fermyon/spin/blob/a09fb1f2b6f9a52dee7dbcbe54e6f9ff41a72ea0/crates/loader/src/local/mod.rs#L142-L144

It would be extremely helpful to have this implemented in order to more easily reuse modules across applications.