First round of trimming annotation work

[roji]

To make basic save/query scenarios work with aggressive trimming, as long as user types are properly preserved. This is a conservative/safe wave of annotations that generally avoids risky suppressions; many warnings are still emitted by the linker, but basic scenarios still work. I've kept non-annotation code changes to a minimum, but I did some simple ones were done.

Here are some details and numbers; all scenarios were executed with the modified trimming tests we run in CI.

Trimming mode	EF size	Total publish dir size	Notes
No trimming	4588kb	93MB
Partial	4588kb	54MB	Default before 7.0, ASP.NET default in 7.0
Aggressive (before this PR)	380kb	34MB	Application fails.
Aggressive (after this PR)	3384kb	34MB	Console app default starting with 7.0. Application works.

To summarize, aggressive trimming mode cuts down 1204kb of pure EF size, totalling a 26% reduction in size. Precise per-assembly sizes are listed below.

EF sizes without trimming

28      Microsoft.EntityFrameworkCore.Abstractions.dll
24      Microsoft.EntityFrameworkCore.Analyzers.dll
2144    Microsoft.EntityFrameworkCore.dll
1904    Microsoft.EntityFrameworkCore.Relational.dll
488     Microsoft.EntityFrameworkCore.SqlServer.dll
4588    total

EF sizes after aggressive trimming (before this PR)

8       Microsoft.EntityFrameworkCore.Abstractions.dll
220     Microsoft.EntityFrameworkCore.dll
116     Microsoft.EntityFrameworkCore.Relational.dll
36      Microsoft.EntityFrameworkCore.SqlServer.dll
380     total

EF sizes after aggressive trimming (after this PR)

12      Microsoft.EntityFrameworkCore.Abstractions.dll
1636    Microsoft.EntityFrameworkCore.dll
1432    Microsoft.EntityFrameworkCore.Relational.dll
304     Microsoft.EntityFrameworkCore.SqlServer.dll
3384    total

Closes #29092

/cc @DamianEdwards after this, the TrimmedTodo console sample in https://github.com/DamianEdwards/TrimmedTodo works in .NET 7.0 even without specifying partial trimming.