Add basic RawSpan support for initializing and merging messages

[pawelmajcher]

On platforms targeting Swift 6.2 and later this allows a RawSpan in lieu of SwiftProtobufContiguousBytes-conforming type when creating or merging a protobuf message. This will improve memory usage by reducing allocations, for example when parsing a large document that includes protobuf messages as part of its binary. Unlike the internal _merge function, this does not expose unsafe API while retaining its benefits.

• I decided to make a twin declaration of the initializer and merge function instead of conforming RawSpan to SwiftProtobufContiguousTypes, because
  • a) RawSpan is not mutable, and
  • b) Span types are non-owning and cannot conform to initializers.
• I used the conditional compilation block and @available directive per analogy to this implementation in SwiftNIO docs.

[Lukasa]

As an API this makes sense to me, but I'd like to see some tests here.

[pawelmajcher]

I added test coverage for both initializing and merging messages with RawSpan, but I'm not sure if more extensive testing is needed here.

[Lukasa]

I think for now this is totally sufficient. Thanks so much!