Add ConfigureNpgsqlDataSourceBuilder for Npgsql plugin registration

[jeremydmiller]

Summary

• New public API: StoreOptions.ConfigureNpgsqlDataSourceBuilder(Action<NpgsqlDataSourceBuilder>)
• Wraps the builder factory so Npgsql plugins (UseVector(), UseNodaTime(), UseNetTopologySuite(), etc.) are applied to every NpgsqlDataSource Marten creates, including tenant databases
• Foundation for the external Marten.PgVector extension
• Resolves the Npgsql type mapping aspect of Installing an extension that defines extra types results in types not being found #2515

Usage

opts.ConfigureNpgsqlDataSourceBuilder(b => b.UseVector());
opts.Storage.ExtendedSchemaObjects.Add(new Extension("vector"));

Test plan

• Marten core compiles
• Verified end-to-end with 14 pgvector integration tests across single tenancy, conjoined tenancy, and database-per-tenant (in CritterWatch tools/Marten.PgVector)

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