Add database migration playground and skip connection string validation for EF tooling

[JamesNK]

Fixes #398

Improves Aspire + EF migrations:

1. Centralized connection string validation in a shared file. The same code and message were duplicated everywhere.
2. Updated connection string validation for Aspire EF database components to be skipped if EF.IsDesignTime is true.
3. Adds a migration focus project to solution. Is configured to apply EF migrations in a project that starts with the host. The readme.md explains how to use it.

Microsoft Reviewers: Open in CodeFlow

[JamesNK]

FYI @roji @ajcvickers @SamMonoRT @AndriySvyryd

[mitchdenny]

Can you file a bug with EF for this and add a link to it in an XML comment?

[AndriySvyryd]

It's tracked by dotnet/efcore#23853

[mitchdenny]

This looks like a workable solution to me. Good stuff!

[eerhardt]

Does this need a Version.Details.xml entry?

[JamesNK]

I don't know. I'm not familiar with when it needs to be updated.

[eerhardt]

cc @joperezr

[eerhardt]

Can we just do this in a Directory.Build.props in the playground\DatabaseMigration folder instead of having this duplicated across all the new projects?

[JamesNK]

I tried doing that, but it introduced a new problem with the Aspire project not able to find its props/targets.

BaseIntermediateOutputPath change only needs to be done inside the database migration example.

[eerhardt]

but it introduced a new problem with the Aspire project not able to find its props/targets.

I don't understand what this means. Can you elaborate?

BaseIntermediateOutputPath change only needs to be done inside the database migration example.

Right, that's why I suggest adding it to the playground\DatabaseMigration folder.

[JamesNK]

Now it works #2327 🤷