Add Utf8String skeleton (dotnet/coreclr#23209)

Add Utf8String skeleton (dotnet/coreclr#23209)

Utf8String is an experimental type that is string-like (heap-allocated, immutable, variable-length, null-terminated) but whose inner representation is UTF-8, not UTF-16.

This is a skeleton implementation of the basic API shape. The ecosystem of APIs has not yet been built around it. All Utf8String-related code is currently surrounded by ifdefs to allow easy identification and removal from release branches.

Commit migrated from https://github.com/dotnet/coreclr/commit/1f3f474a13bdde1c5fecdf8cd9ce525dbe5df000