Support DbDataSource

[roji]

This provides the basic support, allowing:

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    => optionsBuilder.UseNpgsql(/* some data source instance */)

... or with DI:

var appServiceProvider = new ServiceCollection()
    .AddNpgsqlDataSource("Host=localhost;Username=test;Password=test")
    .AddDbContext<BlogContext>(o => o
        .UseNpgsql()
        .LogTo(Console.WriteLine, LogLevel.Information)
        .EnableSensitiveDataLogging())
    .BuildServiceProvider();

Notably excluded are:

• Use of the data source's type mapper instead of the global one (depends on Implement type mapping plugins at the NpgsqlDataSource level (and remove connection-level) npgsql#4494).
• Use of data source (connection-less) commands, to unlock multiplexing (depends on Use DbDataSource.CreateCommand when a transaction isn't necessary dotnet/efcore#28267)

Part of #2400

/cc @ajcvickers @NinoFloris @vonzshik

[NinoFloris]

Safer to do the following

Suggested change

	?? options.FindExtension<CoreOptionsExtension>()?.ApplicationServiceProvider?.GetService<DbDataSource>();
	?? options.FindExtension<CoreOptionsExtension>()?.ApplicationServiceProvider?.GetService<NpgsqlDataSource>();

[roji]

The thing is that we want to support a wrapping data source as well, which wouldn't be an NpgsqlDataSource.

Basically I think that if a DbDataSource is registered in DI, and the user hasn't specified a connection string or connection in UseNpgsql, it makes sense to pick that up. What do you think?

[NinoFloris]

I think it's better to leave the wrapping DbDataSource scenario to manual config. The reason I gave the suggestion is that you don't want to blow up down the line because DbDataSource is actually of type SqliteDataSource and it doesn't quite work the same.

[roji]

But wouldn't that happen only if you don't otherwise explicitly configure EF (i.e. with a connection string), meaning that it will blow up anyway?

[roji]

Discussed offline, we'll start by only resolving NpgsqlDataSource from DI, to minimize confusion in case an incompatible data source is present. Users who want wrappers can just use the explicit option with UseNpgsql.