Consider switching to extension properties for our metadata extension methods

We define a great number of extension methods over our metadata; these can be converted to extension properties with the new C# 14 [extension members](https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-14#extension-members). The extension in question are mostly provider-facing (or possible advanced user-facing), so the breaking change aspect probably isn't too serious (we can also deprecate the methods first).

/cc @AndriySvyryd