Add `CGDataProvider` initializers

[madsmtm]

core-graphics has several functions for initializing CGDataProvider, see the source (not actually exposed, since the type is #[doc(hidden)] for some reason). See also upstream issue servo/core-foundation-rs#185.

It might be desirable to provide similar functionality in objc2-core-graphics, though it's a bit difficult to choose the correct abstraction; do we take &'static [u8], Box<[u8]>, Vec<[u8]> or Arc<[u8]>? Inside smart pointers, do we accept any impl AsRef<u8> + Send + Sync + 'static? Do we provide from_slice_unchecked?

We could of course do all of these, but I'd like to know whether the demand is high enough to be worth it.

[madsmtm]

One use-case: softbuffer uses Box<[u8]>, see https://github.com/rust-windowing/softbuffer/blob/ba60228f4b7bea6986c67288b29c6320f6c02050/src/backends/cg.rs#L293-L315