Allow providing a directory of static files

[seqre]

Currently, our static_files! macro allows only to provide a single file at a time, which doesn't scale well. We should expand it with the possibility to provide a directory, from which all files should be ingested as static ones.

[m4tx]

For embedding the static files in the server binary, this is blocked on rust-lang/rust#99515. Currently, when we read a directory in a proc macro and the directory content changes, there's no way to force recompile, resulting in stale assets.

For having "dynamic static files", where the assets are not embedded, but rather read from a directory at runtime, this is much more viable right now.