Add APIs for registering keyed EF services in DI

.NET 8.0 brings [keyed services](https://learn.microsoft.com/en-us/dotnet/core/whats-new/dotnet-8#keyed-di-services) to DI, allowing multiple services with the same CLR type to be registered in the same service provider. This can be useful for various scenarios, e.g. a read-only and read-write DbContext (same CLR type, different connection strings), possibly restricted multitenancy scenarios, and so on.

This could potentially also be useful for some scenarios in EF's internal service provider...