rust: firmware: add `module_firmware!` macro

Analogous to the `module!` macro `module_firmware!` adds additional
firmware path strings to the .modinfo section.

In contrast to `module!`, where path strings need to be string literals,
path strings can be composed with the `firmware::ModInfoBuilder`.

Some drivers require a lot of firmware files (such as nova-core) and
hence benefit from more flexibility composing firmware path strings.

Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20250306222336.23482-4-dakr@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>