Support `alloc::Vec` in `read_to_end` (and others) with an `alloc` feature

[kleinesfilmroellchen]

Quite a few systems can support alloc even though they’re no_std, allowing use of normal Rust Vec and friends. littlefs2 could provide an optional alloc feature to enable use of alloc functionality with various APIs, most importantly a read_to_end variant that uses alloc::Vec instead of heapless::Vec.

[kleinesfilmroellchen]

In case anyone has the energy for a PR before I do, the method is basically:

	/// Read all remaining data from this file into a provided vector. The vector is resized as needed.
	fn read_all<A: Allocator>(&self, buf: &mut Vec<u8, A>) -> littlefs2::io::Result<()> {
		// figure out how much more there is to read, and resize vector accordingly
		let current_pos = self.seek(littlefs2::io::SeekFrom::Current(0))?;
		let file_length = self.len()?;
		let remaining = file_length - current_pos;
		buf.resize(remaining, u8::default());

		self.read(buf)?;
		Ok(())
	}