Annotate EF Core for nullable reference types

[roji]

Note: #14150 tracks introducing nullability to non-public parts as well (i.e. annotate the entire codebase), and isn't currently planned for 5.0.

This issue now tracks annotating the entire codebase for C# 8 NRTs.

/cc @rynowak @cartermp

Work status

• Analyzers
• Abstractions
• Change tracking (@smitpatel)
• Design (@bricelam)
• Diagnostics
• Extensions
• Infrastructure
• Internal
• Metadata
• Microsoft.Data.Sqlite
• Migrations (@bricelam)
• Proxies
• Query (leftovers in Cosmos, @smitpatel)
• Storage (leftovers in Cosmos, @roji)
• Tools & cmdline (@bricelam)
• Update (@roji)
• ValueGeneration (@roji)

Some specific leftovers we need to do:

• See if we want to somehow pass nullablePublicOnly to the compiler - but we use pubternal extensively. Making EF Core more trimming friendly may be an alternative. See Add nullablePublicOnly Roslyn Feature aspnetcore#29908.
• Take care of MemberIdentity.Name (Annotate metadata for nullability, leaving only builders and conventions #23381, @roji). PR: Obsolete MemberIdentity.IsNone() #24437.
• INavigationBase.GetCollectionAccessor (https://github.com/dotnet/efcore/pull/24261/files#r582596243). There's at least one usage which relies on the null-returning behavior, and it's a public API... so not changing anything, at least for now.
• Search remaining instances of #nullable disable and see if we can get rid of them
• Search remaining instances of TODO-NULLABLE and see if we can get rid of them
• Value converters (temporarily banged in Annotate Storage for nullability #23423, proper annotation should be part of Allow HasConversion/ValueConverters to convert nulls #13850, @ajcvickers)

[ajcvickers]

Consider specifically the scenario in #19443 when working on this.

[ajcvickers]

Tag: C# nullable reference types