Extend app hosting to cover Seq requirements

[nblumhardt]

Seq currently ships Seq.Apps.GenericHost and Seq.Apps.Interrogator binaries for working with .NET app packages. These two executables load and execute app packages, and read app package metadata.

In a near-future Seq release, we'd like to replace GenericHost and Interrogator with corresponding commands from seqcli. The app run command already implements the generic hosting functionality, with some minor feature gaps.

By closing those gaps, and adding a new seqcli app define command to play the role of Interrogator, we can remove the two standalone executables.

This will:

• Reduce the sizes of the Windows installer and Docker image
• Make the development-time app debugging infrastructure (seqcli app run) match the runtime app environment exactly, making behavior around dependent assembly resolution and binding more predictable

There are three parts to this work:

1. This PR: introduce the required commands and features in seqcli
2. A near-future Seq release: let users opt-in to running apps with the new host
3. A Seq release on .NET 5, which will switch all app hosting to the new model, breaking Seq's last .NET Framework dependencies on Windows

Before this can be merged:

• Implement seqcli app define, to read app metadata from .NET attributes
• Add code-level documentation for the app metadata format should be covered in docs
• Add tests for app define
• Allow app run to consume settings from environment variables, so that command-line length limits are avoided, and app configuration can trivially include newlines and other non-CLI-safe characters
• Set the app's id and title correctly in app run
• Migrate any remaining tests from the Seq codebase that cover the app loader, host, and interrogator

[nblumhardt]

This replaces the __MainReactorTypeName top-level property; reasonable to include here because hosted-dotnet is a logical alternative for other platform values (win-x64 etc.)

[KodrAus]

Looks good to me (once the build is 🍏)!