Add rule to seal internal and private types

[BrennanConroy]

Enabling CA1852

This seems like a mostly good change, obviously this first pass will be the worst part, but future work should be simple to react to, the analyzer will just tell you to make your type sealed 😃

Split the changes into commits for each folder in /src, so /SignalR, /Servers, /Components, etc. are all in separated commits.

The one downside is that we like to use "internals visible to" which means some types get flagged by the analyzer to be marked as sealed, but test code inherits from the type, so you need to ignore the warning with:

#pragma warning disable CA1852 // Seal internal types
internal class MyType
#pragma warning restore CA1852 // Seal internal types

[BrennanConroy]

Giving other reviewers until EOD Wednesday PST to give any feedback otherwise merging.