* BTCPay Extensions Part 2
This PR cleans up the extension system a bit in that:
* It renames the test extension to a more uniform name
* Allows yo uto have system extensions, which are extensions but bundled by default with the release (and cannot be removed)
* Adds a tool to help you generate an extension package from a csproj
* Refactors the UI extension points to a view component
* Moves some more interfaces to the Abstractions csproj
* Rename to plugins
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