Allow custom build configurations in MigrationsBundleCommand

[m-x-shokhzod]

Fixes #38193.

Problem

dotnet ef migrations bundle --configuration <Name> only forwarded the configuration to the inner dotnet publish call when the value was Debug or Release. Any other configuration name (e.g. EfBundle) was silently dropped, so the bundle was built with the default configuration instead of the one the user requested.

Root cause

The Debug/Release whitelist was added in 1422c65 ("Bundles: Flow configuration from project", #25906) so that auto-detected project configurations would not flow through unfiltered. However, the same check also blocked user-explicit --configuration values, which by then had already been validated by dotnet publish itself.

Fix

Forward the configuration whenever it is non-empty. dotnet publish is responsible for validating that the configuration name exists in the project; the ef tool should not second-guess it.

 var configuration = Configuration!.Value();
-if (string.Equals(configuration, "Debug", StringComparison.OrdinalIgnoreCase)
-    || string.Equals(configuration, "Release", StringComparison.OrdinalIgnoreCase))
+if (!string.IsNullOrEmpty(configuration))
 {
     publishArgs.Add("--configuration");
     publishArgs.Add(configuration!);
 }

Verification

• Builds clean for both net472 and net10.0 targets of src/ef/ef.csproj.
• Existing test/ef.Tests suite passes (6/6).

The execution path of MigrationsBundleCommand.Execute shells out to dotnet publish and is not unit-tested today; happy to extract the publish-args construction into a testable helper if reviewers prefer.

[AndriySvyryd]

Thanks for your contribution