Add Directory.Packages.props for central package management

[erlendellefsen]

Package versions live in each csproj. Microsoft.EntityFrameworkCore 10.0.7 is duplicated across the library and tests. Move to central package management.

Acceptance criteria

• New Directory.Packages.props with ManagePackageVersionsCentrally=true and one <PackageVersion> per package
• Both csproj files use <PackageReference> without a Version
• dotnet restore --locked-mode works in CI
• No package version regressions