Add `open_protocol_opt()` helpers

[phip1611]

This originated from a discussion in #1858. I argue that uefi-rs should aim for abstractions that expose less and less low-level UEFI details to the user. One example I have in mind is open_protocol_opt() -> Result<Option<ScopedProtocol<P>>>. This helps to better differentiate between "is the protocol just not there" and an actual error.

There are many cases both internal code as well as where peope will use uefi-rs where this would be very benefitial.

This cleanly distinguishes between:

• Ok(None): the protocol is not present
• Ok(Some(_)): the protocol is present and opened successfully
• Err(_): a real error occurred

Eventually, we could deprecate the old versions even.