Support for partial indexes

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https://www.postgresql.org/docs/current/static/indexes-partial.html

There's a PR adding support for SQL Server (and general infrastructure to relational): dotnet/efcore#6866. This covers the PostgreSQL case as well, all we need to do is scaffolding, tests...

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Note that the expression actually given by the user is bound to by database-specific, so maybe it makes sense to only have ForNpgsql, ForSqlServer...