Add AddNpgsqlDataSource with DI service key

[roji]

Npgsql.DependencyInjection allows registering an NpgsqlDataSource in DI. .NET 8.0 is adding support for keyed services (docs); we should add AddNpgsqlDataSource overloads which accept a key. This is ideal for registering multiple data sources in the same DI container, and later selecting which one you want via e.g. [FromKeyedServices].

[adamsitnik]

I would be happy to contribute new methods that use keyed DI, my only question is when is the right time to do that?

Keyed DI was introduced in .NET 8 Preview 7, but it requires RC1 bits (dotnet/runtime#89509 is a must have). RC1 bits are not available at nuget.org, so my question is when should I send the PR: when they become available (few weeks from now) or when .NET 8 is officially shipped (November)?

[roji]

Thanks @adamsitnik! I think once rc1 ships, we're likely to also ship the Npgsql rc1 very soon after that. That point would like be best for doing a PR (so before rc2). Would be great to have a contribution for this!

[roji]

@adamsitnik just in case you're still interested in this :) If not I'll try to do it before GA.