Use Nullable Type Attributes for Properties/Fields where Initialization is Outside the Constructor

Given properties that the compiler can't detect initialization for, such as MSTest's `TestContext` or a calculated `Name` property that splits the assignment to `FirstName` and `LastName`, use nullable attributes.
Examples:
```
[DisallowNull][NotNull]
public TestContext? TestContext {get;set;}
```

and
```
Person(string name) { Name = name; }
[DisallowNull][NotNull]
public string? FirstName {get;set;}
[DisallowNull][NotNull]
public string? FirstName {get;set;}
public string Name
{
get => $"{FirstName} {LastName}";
set => { ... }
}
```

Proposal:
Create a code analysis rule that adds the attributes rather than creates generates a pragma.