Moves the `ViewsRazor` extension into Abstractions, so that it can be used by plugins.
Separated out of #2701, prerequisite for the LNbank plugin integration.
This allows plugins to create custom dbcontexts, which would be namespaced in the scheme with a prefix. Migrations are supported too and the table would be prefixed too
* 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
